Optical transmission system, optical multiplexing transmission system, and related peripheral techniques

ABSTRACT

The invention provides a technique for optimizing transmission conditions to achieve large-capacity transmission, and also provides peripheral techniques for the practical implementation of optical multiplexing that makes large-capacity transmission possible. A transmission characteristic is measured in a transmission characteristic measuring section, and control of signal light wavelength in a tunable light source, control of the amount of prechirping, control of the amount of dispersion compensation, and/or control of optical power are performed to achieve the best transmission characteristic. Wavelength dispersion is deliberately introduced by a dispersion compensator, to reduce nonlinear effects. A tunable laser is used to optimize signal light wavelength for each optical amplification repeater section. Peripheral techniques, such as drift compensation, clock extraction, optical signal channel identification, clock phase stabilization, etc., are provided for the implementation of optical multiplexing.

This application is a division of application Ser. No. 08/781,511 nowU.S. Pat. No. 5,754,322, filed Jan. 8, 1997, which is a continuation ofapplication Ser. No. 08/510,474 now U.S. Pat. No. 5,717,510, filed Aug.2, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical transmission system and itsrelated techniques. More particularly, the invention relates to anoptical transmission system having a transmission line with itstransmission conditions optimized for large-capacity transmission, anoptical transmission system employing an optical multiplexing techniquesuch as optical time-division multiplexing (OTDM) to achievelarge-capacity transmission, and related techniques for implementing thesame.

2. Description of the Related Art

Increasing the transmission speed severely limits the transmissiondistance because of the waveform distortion, caused by group-velocitydispersion (GVD), in optical fibers. Furthermore, when the transmissionspeed is increased, the optical power for transmission needs to beincreased to maintain required difference between transmitted andreceived optical power levels. This in turn increases the effect ofself-phase modulation (SPM), a nonlinear effect of optical fibers, whichfurther complicates the waveform degradation through interaction withgroup velocity dispersion (SPM-GVD effect).

When the waveform distortion caused by the SPM-GVD effect is dominant,the scaling rule expressed by the following equation essentially holds.

    DB.sup.2 P.sub.av L.sup.2 =const.                          (1)

D: dispersion value (ps/nm/km)

B: transmission rate (Gb/s)

P_(av) : average optical power through transmission line (mW)

L: transmission distance (km) p0 const.: determined by required penalty

For example, when the transmission rate B is quadrupled from 10 Gb/s to40 Gb/s, the average optical power P_(av) through the transmission lineneeds to be quadrupled. This means that to achieve the same transmissiondistance, the dispersion value D at signal wavelength must be set to1/64.

To minimize the dispersion value of signal light, work is currentlyunder way to transmit signals in the 1.55-μm range by using adispersion-shifted fiber (DSF), an optical fiber whose zero-dispersionwavelength λ₀ is shifted to the 1.55-μm range where fiber transmissionloss is minimum. The zero-dispersion wavelength λ₀ is the wavelength atwhich the chromatic dispersion value D (ps/nm/km), representing theamount of change of propagation delay time with respect to slightvariations in wavelength, changes from negative (normal dispersion) topositive (abnormal dispersion). Near this wavelength λ₀, the absolutevalue of chromatic dispersion becomes the smallest, so that the waveformdistortion due to the chromatic dispersion is reduced to a minimum.

However, since the fiber drawing process introduces slight variations infiber core diameter, the zero-dispersion wavelength λ₀ of a DSFtransmission line is inevitably subjected to variations along itslongitudinal direction. Furthermore, transmission cables are fabricatedby connecting segments of multi-core cables, each segment extendingseveral kilometers, and the wavelengths λ₀ between adjacent segments arenot continuous but randomly distributed. Moreover, λ₀ varies with agingand due to changes in ambient temperature, etc.

Therefore, in the prior art, the worst-case design has been employed bywhich the transmission line has been designed by considering thedistribution of λ₀ and the deterioration with time so that the requiredtransmission quality can be satisfied even if the worst-case value isapplied throughout the transmission line. This has inevitably increasedtransmission line costs, which has impeded the implementation ofhigh-capacity transmission systems.

On the other hand, signal processing, such as modulation anddemodulation of optical signals, is usually performed at the electricalsignal level, and it has been standard practice to increase the speed ofoptical transmission systems by increasing the speed of electricalsignals used to modulate optical signals. In recent years, however,increasing the speed at the electrical signal level using electronicdevices has been posing a difficult problem. Research and development isbeing undertaken on optical communication devices, at 10 to 40 Gb/s,using Si, GaAs, HBT, HEMT, etc., but it is said that at the presentstate of technology, 10 to 20 Gb/s is the maximum for practicalimplementation.

Therefore, to increase the transmission speed of optical transmissionsystems beyond the operating speeds of electronic devices, multiplexingtechniques in the optical region provide effective means. There are twomain techniques that can be used: one is wavelength-divisionmultiplexing (WDM) and the other is optical time-division multiplexing(OTDM). For practical implementation of either technique, development ofrelated peripheral techniques is needed.

SUMMARY OF THE INVENTION

Accordingly, it is a first object of the invention to provide atechnique for optimizing transmission conditions to achievehigh-capacity transmission.

It is a second object of the invention to provide peripheral techniquesfor the practical implementation of optical multiplexing that makeslarge-capacity transmission possible.

According to the present invention, there is provided an opticaltransmission system comprising: an optical transmitter for generating anoptical signal; an optical transmission line for transmitting theoptical signal generated by the optical transmitter; an optical receiverfor recognizing the optical signal transmitted over the opticaltransmission line; and characteristic adjusting means for adjusting atleast either a characteristic value of the optical signal or acharacteristic value of the optical transmission line, and therebymatching the characteristic of the optical signal to that of the opticaltransmission line.

According to the present invention, there is also provided an opticaltransmission system comprising: an optical transmitter for generating anoptical signal; an optical transmission line for transmitting theoptical signal generated by the optical transmitter; an optical receiverfor recognizing the optical signal transmitted over the opticaltransmission line; and means for reducing a nonlinear effect bysmoothing variation of the power of the optical signal transmittedthrough the optical transmission line.

According to the present invention, there is also provided an opticaltransmission system comprising: an optical transmitter for generating anoptical signal; an optical transmission line for transmitting theoptical signal generated by the optical transmitter; an optical receiverfor recognizing the optical signal transmitted over the opticaltransmission line; an optical amplification repeater, installed at anintermediate point along the optical transmission line, for opticallyamplifying the optical signal being transmitted along the opticaltransmission line; and a wavelength converter for converting thewavelength of the optical signal optically amplified in the opticalamplification repeater.

According to the present invention, there is also provided a driftcompensation circuit for optical modulators in an optical multiplexingsystem in which a plurality of optical signals modulated with basebandsignals by a plurality of optical modulators are multiplexed together,comprising: a plurality of drive circuits for amplitude-modulating thebaseband signals with low-frequency signals before the baseband signalsare supplied to the plurality of optical modulators; an optical couplerfor separating a portion of an optical multiplexed signal generated bymultiplexing the plurality of optical signals; an optical detector forconverting the portion of the optical multiplexed signal separated bythe optical coupler into an electrical signal; and control means forgenerating a bias signal for the drift compensation of each of theoptical modulators by phase-detecting the low-frequency signalcomponents contained in the output of the optical detector with thelow-frequency signals respectively used in the plurality of drivecircuits.

According to the present invention, there is also provided an opticaltransmission system comprising: optical time-division multiplexing meansfor time-division multiplexing a plurality of optical signals; anoptical transmission line for transmitting an optical multiplexed signalgenerated by the optical time-division multiplexing means; clockextraction means for extracting a clock signal for the original opticalsignals directly from the optical multiplexed signal transmitted overthe optical transmission line; and amplitude difference providing meansfor providing amplitude differences among the optical signalsmultiplexed on the optical multiplexed signal to be supplied to theclock extraction means and thereby enabling the clock extraction meansto extract the clock signal.

According to the present invention, there is also provided an opticaltransmitter comprising: optical time-division multiplexing means fortime-division multiplexing a plurality of optical signals; and amplitudedifference providing means for providing amplitude differences among theoptical signals to be multiplexed on an optical multiplexed signal sothat a clock signal for the original optical signals can be extracteddirectly from the optical multiplexed signal at the receiving end.

According to the present invention, there is also provided an opticaltransmission system comprising: optical time-division multiplexing meansfor time-division multiplexing a plurality of optical signal channels;means for appending identification information, for identifying eachoptical signal channel, to an optical multiplexed signal generated bythe optical time-division multiplexing means; an identificationinformation extraction circuit for extracting the identificationinformation contained in the optical signal channel; and a controlcircuit for changing output destinations so that each optical signalchannel is output to a designated destination in accordance with theidentification information extracted by the identification informationextraction circuit.

According to the present invention, there is also provided an opticalreceiver comprising: an identification information extraction circuitfor extracting the identification information contained in the opticalsignal channel; and a control circuit for changing output destinationsso that each optical signal channel is output to a designateddestination in accordance with the identification information extractedby the identification information extraction circuit.

According to the present invention, there is also provided an opticaltransmitter comprising: optical time-division multiplexing means fortime-division multiplexing a plurality of optical signal channels; andmeans for appending identification information for identifying eachoptical signal channel, to an optical multiplexed signal generated bythe optical time-division multiplexing means.

According to the present invention, there is also provided an opticalreceiver for receiving an optical time-division multiplexed signalcarrying a plurality of optical signals time-division multiplexedthereon and having a low-frequency signal superimposed thereon in a timeslot designated for a specific optical signal, comprising: an opticalswitch for separating the optical time-division multiplexed signal intothe respective optical signals; clock generating means for generating aclock signal for controlling the optical switch; and clock phase controlmeans for performing control so that the clock signal generated by theclock generating means is phase synchronized, to the opticaltime-division multiplexed signal, by using the low-frequency signalsuperimposed on the optical time-division multiplexed signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of an optical transmissionsystem according to the present invention, in which a tunable lightsource is provided;

FIG. 2 is a perspective view showing a tunable semiconductor laser as anexample of the tunable light source;

FIG. 3 is a block diagram showing another example of the opticaltransmission system according to the present invention, furtherincluding a tunable filter for each repeater;

FIG. 4 is a block diagram showing another example of the opticaltransmission system according to the present invention, furtherincluding a transmission characteristic measuring section;

FIG. 5 is a diagram for explaining a method of determining wavelengthfrom a measured value of a bit-error rate;

FIG. 6 is a diagram for explaining the measurement of a transmissioncharacteristic using an eye pattern;

FIG. 7 is a diagram for explaining a Q value;

FIG. 8 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 9 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 10 is a diagram showing an example of the optical transmissionsystem according to the present invention, in which a variabledispersion compensator is installed at transmitting end;

FIG. 11 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 12 is a diagram showing an example of the optical transmissionsystem according to the present invention, in which a variabledispersion compensator is installed at receiving end;

FIG. 13 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 14 is a diagram showing an example of the optical transmissionsystem according to the present invention, in which a variabledispersion compensator is also installed at repeater;

FIG. 15 is a diagram showing an example of the optical transmissionsystem according to the present invention, further including atransmission characteristic measuring section;

FIG. 16 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 17 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 18 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 19 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 20 is a diagram showing a characteristic of a Mach-Zehnder opticalmodulator;

FIG. 21 is a diagram for explaining red shift and blue shift in theMach-Zehnder optical modulator;

FIG. 22 is a diagram showing a Mach-Zehnder optical modulator in whichan intensity modulator and a phase modulator are connected in tandem tocontrol the amount of pre-chirping;

FIG. 23 is a diagram showing an example of signal light wavelengthspacing in wavelength multiplexing;

FIG. 24 is a diagram showing the temperature dependence of zerodispersion wavelength λ₀ ;

FIG. 25 is a diagram showing an example of temperature evaluation for anoptical fiber;

FIG. 26 is a diagram showing another example of temperature evaluationfor an optical fiber;

FIG. 27 is a diagram showing another example of temperature evaluationfor an optical fiber;

FIG. 28 is a diagram showing an example of an optical transmissionsystem in which the signal light wavelength is changed on the basis oftemperature evaluation;

FIG. 29 is a diagram showing another example of the optical transmissionsystem according to the present invention;

FIG. 30 is a diagram showing an example of an optical transmissionsystem in which the amount of prechirping is changed on the basis oftemperature evaluation;

FIG. 31 is a diagram showing an example of an optical transmissionsystem in which the amount of dispersion compensation is changed on thebasis of temperature evaluation;

FIG. 32 is a diagram showing an example of an optical transmissionsystem in which a variable dispersion compensator is installed at areceiving end;

FIG. 33 is a diagram showing an example of an optical transmissionsystem in which variable dispersion compensators are installed at both atransmitter and a receiver and also at a repeater;

FIG. 34 is a diagram showing an example of an optical transmissionsystem in which the degree of amplification in an optical amplifier ischanged on the basis of temperature evaluation;

FIG. 35 is a diagram showing an example of an optical transmissionsystem in which the signal light wavelength, the amount of prechirping,the amount of dispersion compensation, and the degree of amplificationare changed on the basis of temperature evaluation;

FIG. 36 is a diagram showing an example of an optical transmissionsystem in which a nonlinear effect is reduced by installing a dispersioncompensator at receiving end;

FIG. 37 is a diagram showing another example of the optical transmissionsystem;

FIG. 38 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed at a transmittingend;

FIG. 39 is a diagram showing another example of the optical transmissionsystem;

FIG. 40 is a diagram showing an example of an optical transmissionsystem in which the compensation amount D in the dispersion compensatorinstalled at a receiving end is set to a positive value;

FIG. 41 is a diagram showing an example of an optical transmissionsystem in which the compensation amount D in the dispersion compensatorinstalled at a receiving end is set to a negative value;

FIG. 42 is a diagram showing an example of an optical transmissionsystem in which dispersion compensators with dispersion amounts withsigns opposite to each other are installed at a transmitting end and areceiving end;

FIG. 43 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is also installed at arepeater;

FIG. 44 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed and further, atransmission characteristic is measured, to optimize the signal lightwavelength;

FIG. 45 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed at a transmittingend;

FIG. 46 is a diagram showing an example of an optical transmissionsystem in which dispersion compensators are installed at both atransmitter and a receiver and also at a repeater;

FIG. 47 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed and, further, atransmission characteristic is measured, to control the amount ofprechirping to an optimum value;

FIG. 48 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed at a transmittingend;

FIG. 49 is a diagram showing an example of an optical transmissionsystem in which dispersion compensators are installed at both atransmitter and a receiver and also at a repeater;

FIG. 50 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed and further, atransmission characteristic is measured, to control the signal lightwavelength and the amount of prechirping to optimum values;

FIG. 51 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed at a transmittingend;

FIG. 52 is a diagram showing an example of an optical transmissionsystem in which dispersion compensators are installed at both atransmitter and a receiver and also at a repeater;

FIG. 53 is a diagram showing an example of an optical transmissionsystem in which a wavelength converter is installed at an opticalamplification repeater;

FIG. 54 is a diagram showing an example of an optical transmissionsystem in which the wavelength is also made variable in the transmitter;

FIG. 55 is a cross-sectional view of a wavelength converting laser as anexample of the wavelength converter;

FIG. 56 is a diagram showing an example of an optical transmissionsystem in which a transmission characteristic is measured to optimizethe signal light wavelength for each optical amplification repeatersection;

FIG. 57 is a diagram showing an example of an optical transmissionsystem in which a dispersion compensator is installed;

FIG. 58 is a diagram for explaining the operation of the driftcompensation circuit when the operating point;

FIG. 59 is a diagram for explaining the operation of the driftcompensation circuit when the operating point has shifted;

FIG. 60 is a diagram for explaining the operation of the driftcompensation circuit when the operating point has shifted;

FIG. 61 is a block diagram showing an example of an optical multiplexingtransmission system having a drift compensation circuit of the presentinvention;

FIG. 62 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 63 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 64 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 65 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 66 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 67 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 68 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 69 is a block diagram showing another example of the opticalmultiplexing transmission system having a drift compensation circuit ofthe present invention;

FIG. 70 is a block diagram showing an example of an opticaltime-division multiplexing system to which a technique of clockextraction according to the present invention is applied;

FIG. 71 is a timing chart for explaining the operation of the systemshown in FIG. 70;

FIG. 72 is a waveform diagram for explaining the clock extractiontechnique of the present invention;

FIG. 73 is a waveform diagram for explaining another example of theclock extraction technique of the present invention;

FIG. 74 is a block diagram showing an example of an optical transmitteraccording to the present invention;

FIG. 75 is a block diagram showing another example of the opticaltransmitter according to the present invention;

FIG. 76 is a block diagram showing another example of the opticaltransmitter according to the present invention;

FIG. 77 is a diagram for explaining how light output strength changeswhen the amplitude of a drive voltage for a Mach-Zehnder opticalmodulator is changed;

FIG. 78 is a diagram for explaining how light output strength changeswhen the bias of a drive voltage for a Mach-Zehnder optical modulator ischanged;

FIG. 79 is a block diagram showing another example of the opticaltransmitter according to the present invention;

FIG. 80 is a block diagram showing an example of an optical receiveraccording to the present invention;

FIG. 81 is a block diagram showing another example of the opticalreceiver according to the present invention;

FIG. 82 is a block diagram showing the details of a clock extractioncircuit;

FIG. 83 is a block diagram showing an example of an opticaldemultiplexer according to the present invention;

FIG. 84 is a diagram showing an example of a transmission data formatcontaining channel identification data;

FIG. 85 is a block diagram showing another example of an opticaldemultiplexer according to the present invention;

FIG. 86 is a block diagram showing another example of an opticaldemultiplexer according to the present invention;

FIG. 87 is a block diagram showing another example of an opticaldemultiplexer according to the present invention;

FIG. 88 is a diagram showing an example of an optical delay circuit;

FIG. 89 is a block diagram showing another example of an opticaldemultiplexer according to the present invention;

FIG. 90 is a diagram showing a low-frequency signal multiplexed on anoptical signal;

FIG. 91 is a block diagram showing another example of an opticaldemultiplexer according to the present invention;

FIG. 92 is a block diagram showing another example of an opticaldemultiplexer according to the present invention;

FIG. 93 is a block diagram showing another example of an opticaldemultiplexer according to the present invention;

FIG. 94 is a block diagram showing an example of an optical transmitteraccording to the present invention;

FIG. 95 is a block diagram showing another example of an opticaltransmitter according to the present invention;

FIG. 96 is a circuit diagram showing the details of drive circuits 418and 420;

FIG. 97 is a block diagram showing an optical receiver in which clockphase stabilization control is performed according to the presentinvention;

FIG. 98 is a timing chart for explaining the operation of the circuitshown in FIG. 97;

FIG. 99 is a diagram for explaining phase difference 0;

FIG. 100 is a diagram showing the relationship between phase difference0 and f₁ component strength;

FIG. 101 is a block diagram showing another example of an opticalreceiver according to the present invention;

FIG. 102 is a block diagram showing another example of an opticalreceiver according to the present invention;

FIG. 103 is a block diagram showing another example of an opticalreceiver according to the present invention;

FIG. 104 is a diagram showing an example of a low-frequency signalsuperimposed on a received signal in the optical receiver shown in FIG.103;

FIG. 105 is a block diagram showing another example of an opticalreceiver according to the present invention;

FIG. 106 is a block diagram showing an example of a detailedconfiguration of a timing regenerator;

FIG. 107 is a waveform diagram showing an example of an opticalmultiplexed signal;

FIG. 108 is a diagram showing a characteristic of an optical detector;

FIG. 109 is a block diagram showing another example of a detailedconfiguration of the timing regenerator;

FIG. 110 is a block diagram showing the details of an optical switch;

FIG. 111 is a block diagram showing another example of an opticalreceiver according to the present invention;

FIG. 112 is a diagram showing the relationship of a synchronousdetection output value relative to the phase difference 0;

FIG. 113 is a diagram showing an example of a circuit for detecting aninput-off alarm condition;

FIG. 114 is a block diagram showing another example of an opticalreceiver according to the present invention;

FIG. 115 is a timing chart explaining an operation of the circuit ofFIG. 86;

FIG. 116 is a block diagram showing an example of an optical exchangeaccording to the present invention;

FIG. 117 is a block diagram showing another example of an opticalexchange according to the present invention;

FIG. 118 is a block diagram showing another example of an opticalexchange according to the present invention;

FIG. 119 is a block diagram showing another example of an opticalexchange according to the present invention; and

FIG. 120 is a waveform diagram showing the operation of the circuit ofFIG. 96.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram showing an example of an optical signaltransmission system according to the present invention. In FIG. 1,reference numeral 11 is an optical transmitter, 12 is an opticalreceiver, 13 is an optical fiber, 14 is tunable light source, 15 is atunable filter, 16 and 17 are optical amplifiers or optical repeatersincluding optical amplifiers, 18 is an optical detector, and 19 is adrive circuit.

The tunable light source 14 used in the optical transmitter 11 can beconstructed, for example, from a tunable semiconductor laser of knownconfiguration, such as a three-electrode type or an external diffractiongrating controlled type.

FIG. 2 is a diagram for explaining a tunable semiconductor laser; thebasic structure of a three-electrode tunable semiconductor laser isshown here. The tunable semiconductor laser shown in FIG. 2 has anInGaAsP/InP laser configuration. As shown, a laser oscillation region 27containing an active layer 25 is formed between a common electrode 21and an electrode 22, a wavelength fine-adjusting region 28 is formedbetween the common electrode 21 and an electrode 23, and a wavelengthcoarse-adjusting region 29 containing a diffraction grating 26 is formedbetween the common electrode 21 and an electrode 24. The emissionwavelength can be varied by adjusting a current Ip applied to theelectrode 23 and a current Id applied to the electrode 24, and theoptical output can be controlled by adjusting a current Ia applied tothe electrode 22. Therefore, by controlling the currents Ia, Ip, and Idvia the drive circuit 19, the emission wavelength can be controlledwhile outputting an optical signal modulated with the information to betransmitted.

Instead of the direct modulation type described above that directlydrives the light source, the tunable light source 14 may be constructedas an external modulation type which uses an external modulator tomodulate the light from the light source. Furthermore, an opticalamplifier may be provided for amplifying an optical signal from thetunable light source 14 for transmission through the optical fiber 13.

The optical amplifiers 16 and 17 can be constructed from optical-fiberamplifiers doped with rare earth elements such as Er, Nd, etc. Forexample an Er-doped optical-fiber amplifier can directly amplify anoptical signal at 1.5 μm when pumped with light at 1.48 μm or 0.98 μm.

The optical receiver 12 consists, for example, of the optical amplifier17, tunable filter 15, and optical detector 18. The optical amplifier 17and the tunable filter 15 may be omitted. The optical detector 18 isconstructed, for example, from a photodiode or a phototransistor ofknown structure for converting an optical signal into an electricalsignal. An information processing section for converting the signal toelectrical form by the optical detector 18, performing equalization forlevel discrimination, and recovering the transmitted information forreception, is not shown here since it can be implemented using variousknown configurations.

The tunable filter 15 can be constructed using a filter of knownconfiguration. For example, it is possible to use atemperature-controlled tunable filter as explained by the title "Studyof a Fabry-Perot wavelength-selective optical filter using Si," B-1055,Proc., Institute of Electronics, Information and CommunicationEngineers, Spring Convention, 1994. It is also possible to use afixed-wavelength filter having a transmission wavelength characteristicthat can cover the entire wavelength variable range of the tunable lightsource 14.

In a system in which the optical transmitter 11 and the optical receiver12 are not separated by a large distance, the optical amplifiers 16 canbe omitted, in which case the optical transmitter 11 will be connectedto the optical receiver 12 by the optical fiber 13 alone. If thereexists a fluctuation in the zero dispersion wavelength along thelongitudinal direction of the optical fiber 13, or if there existvariations in the zero dispersion wavelength of the optical fiber 13between fiber segments constructed by splicing manufactured unit lengthsof optical fibers, the average value of such fluctuation or variation ofthe zero dispersion wavelength is obtained along the entire lengthbetween the optical transmitter 11 and the optical receiver 12, and thetunable light source 14 is so regulated as to emit light at thatwavelength, for example. It should be noted that a signal lightwavelength that provides the best transmission characteristics is notalways the zero dispersion wavelength, as explained later.

When an optical signal is amplified by a rare-earth doped optical-fiberamplifier and fed into an optical fiber for transmission over a longdistance, if the wavelength of the signal light is in an abnormaldispersion region near the zero dispersion wavelength of the opticalfiber, a four-wave mixing (FWM) phenomenon occurs between the signallight and spontaneously emitted light, and because of modulationinstability the spontaneous emission is amplified, degrading the S/Nratio. To prevent this, the emission wavelength is controlled within anormal dispersion region, while avoiding a region near the zerodispersion wavelength of the optical fiber 13.

For a long distance transmission system, the optical amplifiers 16 and17 are usually provided. Since the optical amplifiers 16 and 17 not onlyamplify optical signals but emit light as spontaneous emission, it isdesirable to provide a filter in the front stage of the optical receiver18. It is further desirable that the filter be constructed from thetunable filter 15 whose transmission wavelength characteristic isadjustable. In that case, when the emission wavelength of the tunablelight source 14 of the optical transmitter is controlled in the abovemanner, the transmission characteristics can be further improved bycontrolling the transmission wavelength characteristic of the tunablefilter 15 in such a manner as to match the emission wavelength.

FIG. 3 is a block diagram showing another example of the optical signaltransmission system according to the present invention. In FIG. 3,reference numeral 30 is a repeater, 31 is an optical transmitter, 32 isan optical receiver, 33 is an optical fiber, 34 is a tunable lightsource, 35 is a tunable filter, 36 and 37 are optical direct amplifiers,38 is an optical detector, 39 is a drive circuit, and 40 is a tunablefilter. The parts with the same names as those in the embodiment shownin FIG. 1 have the same functions as the corresponding parts alreadydescribed.

In the example shown here, the optical amplifier 36, such as arare-earth doped optical-fiber amplifier, and the tunable filter 40constitute the repeater 30. The wavelength transmission characteristicof the tunable filter 40 in each repeater 30, as with the tunable filter35 in the optical receiver 32, is adjusted so as to block lightcomponents other than the optical signal, such as spontaneous emissionfrom the optical amplifier 36, thus improving transmissioncharacteristics and making long-distance transmission possible. It isalso possible to use, instead of the tunable filter 40, afixed-wavelength filter having a transmission wavelength characteristicthat can cover the entire wavelength variable range of the tunable lightsource 34. Furthermore, in the configuration shown here, the tunablefilter 40 is provided after the optical amplifier 35, but it will berecognized that the invention is not limited to this particularconfiguration.

FIG. 4 is a block diagram showing a further example of the opticalsignal transmission system according to the present invention. In FIG.4, reference numeral 41 is an optical transmitter, 42 is an opticalreceiver, 43 is an optical fiber, 44 is a tunable light source, 45 is atunable filter, 46 and 47 are optical amplifiers, 48 is an opticaldetector, 49 is a drive circuit, 50 is a tunable filter, 51 is arepeater, 52 is a sweep controller, and 53 is a transmissioncharacteristic measuring section.

This example is identical to the embodiment shown in FIG. 3, except thatthe transmission characteristic measuring section 53 and the sweepcontroller 52 are added. The drive circuit 49 is controlled by the sweepcontroller 52 to sweep the emission wavelength of the tunable lightsource 44. For example, when the tunable light source 44 is constructedfrom the tunable semiconductor laser shown in FIG. 2, sweeping can beaccomplished by varying the currents Ip and Id; in the case of asemiconductor laser of other configuration, sweeping of the emissionwavelength can be accomplished by continuously varying the temperature.The optical signal with the thus swept emission wavelength istransmitted along the optical fiber 43 and via the repeaters 51, and isdetected by the optical detector 48 of the optical receiver 42, wherethe received result is applied to the transmission characteristicmeasuring section 53 which measures the transmission characteristicbetween the optical transmitter 11 and the optical receiver 12. Based onthe result of the transmission characteristic measurement, the emissionwavelength of the tunable light source 44 and the wavelengthtransmission characteristics of the tunable filters 45 and 50 are so setas to achieve the best transmission characteristic.

When the system is constructed so that the tunable light source 44 andthe tunable filters 45 and 50 are automatically controlled by thetransmission characteristic measuring section 53, sweep controlinformation for controlling the sweeping of the emission wavelength ofthe tunable light source via the drive circuit 49 is transmitted fromthe sweep controller 52 to the transmission characteristic measuringsection 53, as shown by a dotted line. Based on the sweep controlinformation thus transmitted, the transmission characteristic measuringsection 53 controls the transmission wavelength characteristic of thetunable filters 45 and 50, as shown by dotted lines in the figure. Fromthe transmission characteristic at the optical detector 48 during sweepcontrol, the transmission characteristic measuring section 53 determinesthe emission wavelength that maximizes the transmission characteristic,and sends control information to the drive circuit 49 to control thewavelength to the determined value, while applying control informationto the tunable filters 45 and 50 to control their transmissionwavelength characteristics to match that emission wavelength.

Therefore, by activating the sweep controller 52 when the system isstarted the emission wavelength of the tunable light source 44 can beautomatically set to a value that provides the best transmissioncharacteristics. Further, while the system is in operation, the sweepcontroller 52 may be deactivated, but it would be possible to have thetransmission characteristic measuring section 53 measure thetransmission characteristic periodically or continuously and adjust theemission wavelength of the tunable light source 44 and the wavelengthtransmission characteristics of the tunable filters 45 and 50 so thatthe best transmission characteristic can be achieved.

The control signal for adjusting the wavelength transmissioncharacteristic from the transmission characteristic measuring section 53to each repeater 51 and the control signal applied to the drive circuit49 for optimizing the transmission characteristic or the sweep controlsignal from the sweep controller 52 to the transmission characteristicmeasuring section 53 can be transmitted using a relatively low-speedtransmission line. Therefore, such information can be transmitted overcontrol lines or the like installed between the optical transmitter 11and the optical receiver 12, and in the case of a system in whichoptical signals are transmitted in both directions, such information canbe transmitting by superimposing it as a sub-signal on the main opticalsignal.

When the transmission characteristic measuring section 53 is constructedto measure the transmission characteristic by measuring bit-error rates,if the bit-error rate at each wavelength is measured while sweeping theemission wavelength of the tunable light source 44 and the measurementsare plotted as shown in FIG. 5 where the allowable error rate is set at10⁻¹¹, for example, the emission wavelength of the tunable light source44 is set approximately at the center of the wavelength range fallingwithin the allowable error rate. That is, the drive circuit 49 iscontrolled to set the emission wavelength of the tunable light source 44to the best point of the transmission characteristic, and at the sametime, the wavelength transmission characteristics of the tunable filters45 and 50 are set accordingly. The bit-error rate can thus be controlledwithin the allowable value despite the variation of the characteristicsof the optical fiber 43 caused by temperature changes, aging, etc.

The bit-error rate can be measured using an error rate measuring meansprovided in an ordinary transmission system. If the transmission speedof optical signals is higher than 10 Gb/s, for example, the error rateat each wavelength can be measured in a short time for allowable errorrates smaller than 10⁻¹¹. It is also possible to append a parity checkbit for transmission and measure the error rate using the parity checkbit.

The transmission characteristic measuring section 53 may be constructedto measure the transmission characteristic using an eye pattern. FIG. 6is a diagram for explaining an eye mask pattern. If the eye mask patternshown by thick lines represents a threshold value, for example, theemission wavelength of the tunable light source 44 is adjusted so thatthe eye pattern of the received signal is formed outside the thick-linedeye mask pattern, that is, the eye pattern exceeds the predeterminedthreshold value. Since the eye pattern opens wide when the transmissioncharacteristic is good, the emission wavelength of the tunable lightsource 44 may be adjusted so that the eye pattern opens widest. As anadjusting means in this case, control may be performed manually whileobserving the eye pattern, or alternatively, automatic control by meansof computer processing may be employed.

An alternative way to measuring the bit-error rate is to measure the Qvalue (electrical SNR). The definition of the Q value is given below inconjunction with FIG. 7.

Q=20 log₁₀ (μ₁ -μ₀)/(σ₁ +σ₀)!

where

μ₁ : average level during "emission"

μ₀ : average level during "no emission"

σ₁ : standard deviation of level during "emission"

σ₀ : standard deviation of level during "no emission"

The Q value is expressed using the signal level difference (=signalamplitude) between emission and no emission as the numerator and the sumof the standard deviations of noise during emission and during noemission as the denominator. When a Gaussian distribution is assumed forthe noise distribution, the bit-error rate given by the Q value definedby the above equation agrees with the minimum value of the actuallymeasured bit-error rate. A Q value measuring system is substantiallyidentical in configuration to the optical receiver; that is, using adiscrimination circuit having a reference voltage varying function, thediscrimination level of the equalizing waveform is varied up and downwith respect to the optimum level to measure the bit-error rate, and byfinding the intersection of the two straight lines obtained from themeasurement, the minimum point of the bit-error rate is estimated, toobtain the Q value. Other methods, such as measuring the transmittedwaveform and using specifications of equal bit-error rate curves, mayalso be employed.

FIG. 8 is a block diagram showing another example of the optical signaltransmission system according to the present invention. In FIG. 8,reference numeral 61 is an optical transmitter, 62 is an opticalreceiver, 63 is an optical fiber, 64 is a tunable light source, 65 is anoptical coupler, 66a and 66b are external modulators, 67a and 67b aredrive circuits, 68 is an optical multiplexer, 69 and 70 are opticalamplifiers, 71 is a tunable filter, 72 is an optical coupler, 73a and73b are optical detectors, and 74 is a transmission characteristicmeasuring section.

As in the foregoing examples, the tunable light source 64 can beimplemented using, for example, a tunable semiconductor laser. Theoptical coupler 65 splits the output light of the tunable light source64 into two output signals which are respectively applied to theexternal modulators 66a and 66b. The optical coupler 65 may beconstructed to split the light into three or more output signals todrive the respective external modulators.

Clock signals CLKa and CLKb and transmission information not shown areapplied to the drive circuits 67a and 67b which then generate modulationsignals synchronized to the clock signals CLKa and CLKb and apply themto the external modulators 66a and 66b for modulation of the two splitlight signals. The modulated light signals are multiplexed by theoptical multiplexer 68 and amplified by the amplifier 69 fortransmission through the optical fiber 63. The external modulators 66aand 66b can be constructed, using, for example, Mach-Zehnder opticalmodulators using LiNbO₃ substrates or semiconductor electro-absorptionoptical modulators.

Multiplexing of the optical signals in the optical multiplexer 68 can beaccomplished using various multiplexing means such as bit multiplexing,byte multiplexing, frame multiplexing, etc. Modulation timing in theexternal modulators 66a and 66b is selected according to suchmultiplexing means, and control is performed so that the externalmodulators 66a and 66b output modulated optical signals with differentphases which are then multiplexed together in the optical multiplexer68.

If the optical coupler 65 is constructed, for example, as a Mach-Zehnderoptical modulator having two separate output ports, and the output lightof the tunable light source 64 is input to the optical modulator formodulation with a 10 GHz clock signal, for example, optical clocksignals 180° out of phase with respect to each other are output at 10GHz from the two respective output ports and applied to the externalmodulators 66a and 66b, respectively. These optical signals aremodulated by the external modulators 66a and 66b with information to betransmitted, and optically multiplexed by the optical multiplexer 68which outputs a multiplexed optical signal at a transmission rate of 20Gb/s.

When two separate information signals are transmitted together bytime-division multiplexing, it would be possible at the optical receiver62 to demultiplex them after first converting the multiplexed signal toan electrical signal; in the above example, however, the multiplexedsignal is first amplified by the optical amplifier 70, then passedthrough the tunable filter 71 for elimination of noise and otherunwanted light components, and then fed into the optical coupler 72where the input signal is split into two signals which are then appliedto the respective optical detectors 73a and 73b. Using a clock signalfrom a clock regenerator not shown, two clock signals corresponding tothe clock signals CLKa and CLKb used in the optical transmitter 61 areobtained, and using the thus obtained clock signals, the two transmittedinformation signals are reconstructed from the output signals from theoptical detectors 73a and 73b.

A transmission characteristic measuring section 74 may be provided foreach of the optical detectors 73a and 73b, but in the above example, oneis provided for one of the detectors. When the system is started orwhile the system is in operation, the transmission characteristicmeasuring section 74 measures the transmission characteristic, sets theemission wavelength of the tunable light source 64 for the besttransmission characteristic, and sets the transmission wavelengthcharacteristic of the tunable filter 71 accordingly. This facilitateshigh-speed transmission over long distances. The optical amplifier 69,tunable filter 71, etc. may be omitted.

FIG. 9 is a block diagram showing another example of the opticaltransmission system according to the present invention. In FIG. 9,reference numeral 81 is an optical transmitter, 82 is an opticalreceiver, 83 is an optical fiber, 84 is a tunable light source, 85 is anoptical coupler, 86a and 86b are external modulators, 87a and 87b aredrive circuits, 88 is an optical multiplexer, 89 is an optical directamplifier, 90 is an optical coupler, 91a and 91b are optical directamplifiers, 92a and 92b are tunable filters, 93a and 93b are opticaldetectors, and 94 is a transmission characteristic measuring section.

The optical transmitter 81 is identical, in both configuration andoperation, to the transmitter 61 in the foregoing embodiment. At theoptical receiver 82, the optical signal received via the optical fiber83 is split by the optical coupler 90 into two signals which are thenamplified by the respective optical amplifiers 91a and 91b and appliedto the optical detectors 93a and 93b through the tunable filters 92a and92b, respectively. The two transmitted information signals are thusprocessed by the respective optical detectors 93a and 93b.

As in the foregoing embodiment, the transmission characteristicmeasuring section 94 measures the transmission characteristic betweenthe optical transmitter 81 and the optical receiver 82 by using theoutput signal from one or other of the optical detectors 93a, 93b, andcontrols the emission wavelength of the tunable light source 84 for thebest transmission characteristic, while controlling the wavelengthtransmission characteristics of the tunable filters 92a and 92baccordingly. High-speed transmission is thus made possible bytime-division multiplexing of optical signals; furthermore, even thoughthere exists a variation in the zero dispersion wavelength, since theemission wavelength is controlled so as to give the best transmissioncharacteristics, the transmission distance can be extended.

It will be appreciated that the present invention is not limited to theabove illustrated examples, but various modifications can be made in theinvention. For example, in the embodiments shown in FIGS. 8 and 9, theoutput light of the tunable light source is divided into two signals,but it would be possible to divide it into a larger number of signalswith a corresponding number of external modulators provided so thatoptical signals modulated with transmitted information from multiplelines would be time-division multiplexed, thereby making transmission athigher speeds possible. Further, the above embodiments have each dealtwith an example in which bit multiplexing is used, but it would bepossible to use other multiplexing means such as byte multiplexing orframe multiplexing. Furthermore, in the embodiments shown in FIGS. 8 and9, optical amplifiers may be provided at prescribed intervals ofdistance along the optical fiber 63, 83 to further extend thetransmission distance.

Any of the examples thus far described use a tunable light source andoptimize the transmission conditions by controlling the wavelength ofthe signal light to an optimum value with respect to the transmissionline in use. Conversely, it is possible to use a signal light of a fixedwavelength, in which case the transmission conditions are optimized forthat fixed wavelength by using a variable dispersion compensator capableof adjusting the amount of wavelength dispersion. Examples of such anoptical transmission system will be described below.

In the examples shown in FIGS. 10 and 11, the variable dispersioncompensator is provided at the transmitting end, while in the examplesshown in FIGS. 12 and 13, it is provided at the receiving end. FIGS. 10and 12 each shown an example of a repeaterless transmission system, andFIGS. 11 and 13 each shown an example of a multiple optical amplifierrepeated system. In the figures, reference numeral 100 is an opticaltransmitter, 101 is a variable dispersion compensator capable of varyingthe amount of dispersion, 102 is a transmission line, 103 is an opticalreceiver, and 104 is a repeater amplifier. In the description below, thesame reference numerals indicate the same constituent parts.

The variable dispersion compensator 101 used here can be constructedfrom a Mach-Zehnder interferometer-type dispersion compensator using aplanar lightwave circuit (PLC) (for example, Takiguchi et al.,"Dispersion Compensation Experiments Using PLC-type Light DispersionEqualizer," C-337, Institute of Electronics, Information andCommunication Engineers, Spring Convention, 1994), or an opticalresonator-type dispersion compensator (for example, Fukashiro et al.,"Study on a Dispersion Compensation Method Using an Optical Resonator,"B-935, Institute of Electronics, Information and CommunicationEngineers, Autumn Convention, 1994).

FIG. 14 shows an example of a multiple optical amplifier repeated systemin which a variable dispersion compensator is also provided in eachrepeater. The invention, however, is not limited to the configurationshown in the example of FIG. 14 where both the transmitter and receiveras well as all the repeaters are provided with variable dispersioncompensators, but various other configurations are possible, forexample, a configuration where only the repeaters are provided withvariable dispersion compensators, a configuration where the transmitterand the repeaters are provided with variable dispersion compensators, aconfiguration where the repeaters and the receiver are provided withvariable dispersion compensators. Furthermore, in the case where therepeaters are provided with the variable dispersion compensators, onlysome of the repeaters may be provided with them.

As for the dispersion compensation techniques used in the examples ofFIGS. 10 to 14, though various dispersion compensators and dispersioncompensation methods using them have already been proposed andimplemented for land systems, undersea systems, repeaterless systems,and multiple repeater systems, the point of the present invention isthat using a variable dispersion compensator capable of varying theamount of dispersion, the amount of dispersion compensation is optimizedat a value that maximizes the transmission characteristic for eachrepeater section.

For optimization, when the zero dispersion wavelength of thetransmission line, including the variation along the longitudinaldirection, is known, the optimum dispersion compensation amount can bedetermined through simulation, and this amount is set in the variabledispersion compensator 101.

FIG. 15 shows another example. In this example, sweeping the dispersioncompensation amount is performed while measuring the transmissioncharacteristic at the receiving end, and the variable dispersioncompensator is set to a value at which a good transmissioncharacteristic is obtained. In this example, since the variabledispersion compensator 101 is provided at the receiver, the transmissioncharacteristic is measured at the receiving end while sweeping thedispersion compensation amount, and the optimum dispersion compensationamount is set accordingly. The same method of measurement as used in thepreviously described transmission characteristic measuring section 53,74, or 94 may be used in the transmission characteristic measuringsection 105 for measurement of the transmission characteristic.

FIGS. 16 and 17 show a still further example. In the opticaltransmission system of this example, a control signal is fed back to thetransmitter or to the repeaters on the basis of the transmissioncharacteristic measured at the receiver, to optimize the dispersioncompensation amount that is set in the variable dispersion compensator101 installed therein. FIG. 16 shows a configuration where the variabledispersion compensator is installed only in the transmitter. In thisconfiguration, the transmission characteristic is measured at thereceiving end while the dispersion compensation amount is being swept atthe transmitting end, and the resulting information is fed back so thatthe optimum dispersion compensation amount can be set. FIG. 17 shows aconfiguration where the variable dispersion compensator is installed inthe transmitter, the receiver, and in every repeater. In a system wherea plurality of dispersion compensators are installed, all thecompensators need not necessarily be of dispersion variable type butsome may be constructed from fixed-type dispersion compensators. Afixed-type dispersion compensator can be implemented using a dispersioncompensating fiber (DCF).

The dispersion compensation amount can be controlled by the variabledispersion compensator while monitoring the transmission characteristicnot only when the system is started up but also while the system is inoperation. In this way, proper control can be performed independently ofthe variation of the wavelength of the light source LD and the variationof the zero dispersion wavelength of the transmission line due totemperature changes and aging.

This process may be performed manually or may be automatically performedby the CPU. Furthermore, a separate CPU may be provided for each of theregenerative repeater sections between the optical transmitter andreceiver for independent control, or alternatively, control may beperformed centrally by a single CPU while adjusting the relationshipsamong the plurality of regenerative repeater sections.

FIGS. 18 and 19 show a yet further example. In this example, thevariable dispersion compensator is used in conjunction with a tunablelight source 106. FIG. 18 shows a configuration for repeaterlesstransmission system, and FIG. 19 a configuration for an opticalamplification multiple repeater transmission system. The invention,however, is not limited to the illustrated configuration where thevariable dispersion compensator is installed in the transmitter, thereceiver, and in every repeater, but various combinations are possibleas mentioned with reference to FIG. 14.

In this optical transmission system, the variable dispersion compensatoris used in conjunction with the tunable light source 106 in thetransmitter, and while measuring the transmission characteristic at thereceiver, the transmission wavelength is swept and set to a value atwhich good transmission characteristic is obtained, or based on thetransmission characteristic measured at the receiver, a control signalis fed back to the transmitter so that the wavelength of the tunablelight source 106 is set to the optimum value.

As previously described, when transmission (both repeaterlesstransmission and multiple repeater transmission) is performed at arelatively high optical power level using an optical amplifier, if thewavelength of signal light is set near the zero dispersion wavelength ofthe optical-fiber transmission line and in an abnormal (positive)dispersion region, modulation instability occurs due to a four-wavemixing (FWM) phenomenon that occurs between the signal light and theamplified spontaneous emission (ASE). As a result, the ASE is amplifiedand the S/N ratio of the signal light degrades. It is known that thiscan be effectively avoided by setting the signal light wavelength withinthe normal (negative) dispersion region or by applying positivedispersion at the receiver or at the repeater. That is, dispersioncompensation is performed by setting the wavelength of the tunable lightsource to a value that makes the dispersion value negative with respectto the transmission line and that can suppress FWM, while at the sametime, setting the dispersion amount of the dispersion compensator to apositive value. Alternatively, dispersion compensation may be performedby setting the wavelength of the tunable light source to a value thatmakes the dispersion value positive with respect to the transmissionline and that can suppress FWM, while at the same time, setting thedispersion amount of the dispersion compensator to a negative value. Thesetting of the transmission wavelength may be done automatically. Also,the setting of the transmitting wavelength may be done when the systemis started up, or while the system is in operation.

If there is a spare line whose dispersion condition and installationenvironment are approximately equal to those of the operating line, itwill be possible to first optimize the dispersion compensation amountand transmission wavelength on the spare lone and then apply theoptimization to the operating line by reference to them. This enablesthe optimization of the respective quantities to be done withoutaffecting the service speed.

Parameters for controlling the conditions of the transmission line mayalso include the prechirping amount and the optical power to be input tothe fiber in addition to the signal light wavelength (FIGS. 1, 3, 4, 8,9, 18, 19) and dispersion compensation amount (FIGS. 10 to 19) alreadydescribed.

Prechirping is a technique for controlling the change of the transmittedwaveform due to wavelength dispersion and nonlinear effects bypreproviding a wavelength (frequency) distribution within one pulse ofthe transmission wavelength, and various methods have been proposed forimplementation. When a Mach-Zehnder optical modulator is used as anexternal modulator, the relationship between applied voltage and opticalpower will be represented by a sine curve as shown in FIG. 20. When avoltage near Vb1 is selected and applied as a positive pulse, such asshown in part (1)(a) of FIG. 21, a positive light pulse in phase withthe applied voltage is output, as shown in part (2)(a) of FIG. 21. Atthis time, the light wavelength shifts to the shorter value in therising portion of the light pulse, and to the longer value in thefalling portion, as shown in part (3)(a) of FIG. 21. That is, within theduration of one light pulse, the wavelength shifts with time from theshorter wavelength (blue) to the longer wavelength (red). Thisphenomenon is called a red shift. On the other hand, when a voltage nearVb2 is selected and applied as a negative pulse, such as shown in part(1)(b) of FIG. 21, a positive pulse 180° out of phase with the appliedvoltage is output, as shown in part (2)(b) of FIG. 21. At this time, thelight wavelength shifts to the longer value in the rising portion of thelight pulse, and to the shorter value in the falling portion, as shownin part (3)(b0 of FIG. 21. That is, within the duration of one lightpulse, the wavelength shifts with time from the longer wavelength (red)to the shorter wavelength (blue). This phenomenon is called a blueshift. When a parameter defining the chirping amount is denoted by α,then α>0 for the red shift and α<0 for the blue shift. When thewavelength of the signal light is shorter than the zero dispersionwavelength and the transmission condition of the optical fiber is withinthe region of the normal dispersion (D<0), light at longer wavelengthtravels through the optical fiber faster than light at shorterwavelength; therefore, if a pre-chirping of α>0 (red shift) is given inadvance, this has the effect of compressing the pulse waveform, andsuppresses waveform degradation. Conversely, when the transmissioncondition is within the region of the abnormal dispersion (D>0), lightat shorter wavelength travels faster, so that a prechirping of α<0 (blueshift) will serve to suppress waveform degradation. Further, byadjusting the value of α according to the condition of the transmissionline, the overall transmission condition of the optical system can beoptimized. In a Mach-Zehnder optical modulator, the sign of α can bechanged between positive and negative depending on which operatingpoint, Vb1 or Vb2, is used. When a Mach-Zehnder optical modulator isused that consists of a intensity modulator 107 and a phase modulator108 connected in tandem as shown in FIG. 22, the prechirping amount αcan be varied continuously by varying the voltage being applied to thephase modulator 108. In the illustrated example, the intensity modulatorand the phase modulator are integrated as a single device, but these maybe constructed as separate devices that are connected together.

As for the input optical power to the fiber, by varying the transmitteroptical power and the repeater optical output power, the waveform changedue to the interaction between self-phase modulation and chromaticdispersion in the transmission line can be regulated. In the case of WDMtransmission, the amount of crosstalk (described later) due to FWM canalso be changed by varying the optical power. Varying the optical powercan be easily accomplished by controlling the optical output powers ofthe transmitting light source and of the optical amplifiers (opticalpost-amplifier and in-line optical amplifier).

Control of the prechirping amount and/or control of the optical powercan be performed in place of or in conjunction with control of thesignal light wavelength and/or control of the dispersion compensationamount, in the examples previously shown in FIGS. 1, 3, 4, 8 to 19.

In the examples thus far described, the transmission characteristic ismeasured periodically or continuously, and the control parameters, suchas the signal light wavelength, etc., are adjusted in such a manner asto compensate for the change of the zero dispersion wavelength λ₀ in thetransmission line. One of factors that change the zero dispersionwavelength λ₀ is a temperature change in the transmission line.Regarding the temperature change, the transmission conditions can alsobe optimized by estimating the amount of shift of the zero dispersionwavelength by evaluating the temperature of the transmission line, andby correcting the control parameters based on the estimated shiftamount.

Further, in an optical amplification multiple repeater WDM system usinga wavelength band near the zero dispersion wavelength of the opticalfiber, crosstalk caused by four-wave mixing between signal lightsbecomes a factor that can degrade the transmission characteristic. Toavoid this crosstalk, the signal wavelength band must be spaced far awayfrom the zero dispersion wavelength of the transmission line, contraryto the case of one-wave transmission. An example of the spacing ofsignal light wavelengths is shown in FIG. 23. In this case also, it isimportant to assess the variation of λ₀ along the longitudinal directionof the actual optical-fiber transmission line.

FIG. 24 shows the temperature dependence of λ₀ actually measured.

Source: II. Onaka et al., "Measuring the Longitudinal Distribution ofFour-Wave Mixing Efficiency in Dispersion-Shifted Fibers," IEEEPhotonics Technology Letters, Vol. 6, No. 12, 1994.

In this example, for a DSF of 1.1 km length, λ₀ is obtained from theoccurrence efficiency of four-wave mixing (FWM). Here, a change of 2.4nm (rate of change: 0.03 nm/°C.) is shown over a temperature range of-20° to +60° C. Since the dispersion slope of the DSF used here is 0.07ps/nm² /km, the wavelength dispersion value changes at a rate of 0.168ps/nm/km. At transmission speeds 10 Gb/s or over, this change may haveto be taken into consideration in system design together with thevariation along the longitudinal direction.

Temperature is evaluated at an appropriate point 110 on theoptical-fiber transmission line 102 installed between the opticaltransmitter 100 and the optical receiver 103, as shown in FIG. 25, or atmultiple points as shown in FIG. 26. When temperature is evaluated atmultiple points, the distribution of shift amounts of the zerodispersion wavelength can be obtained. In the case of an opticalamplification repeater transmission system, temperature is evaluated atone point or at multiple points on every repeater section 102 or on someof the repeater sections.

Temperature evaluation can be accomplished by directly measuring thetemperature of the fiber-optic cable of the transmission line using asuitable temperature sensor, or the temperature of the fiber-optic cablecan be estimated by measuring the tube temperature, or in the case of anunderground optical-fiber cable, by measuring the temperature of theearth surface under which the cable is buried, or in the case of anundersea cable, by measuring the water temperature. Furthermore, thecable temperature can be estimated from the ambient temperature at anend station or at a repeater, or from the temperature of the earthsurface. A continuous temperature distribution can be measured by layingan optical fiber for temperature measurement along the fiber-optic cableand by using OTDR (Optical Time Domain Refflectometry) in order tomeasure Raman scattered light.

From the temperature evaluation value thus obtained, the amount ofvariation of λ₀ is computed and based on which, control parameters suchas signal light wavelength are corrected. It is also possible to createcalendars for average seasonal and day-to-night variations from the pasttemperature evaluation results and change the control parameters usingsuch calendars (or data may be preprogrammed).

FIGS. 28 and 29 are concerned with an example in which signal lightwavelength is corrected for each regenerative repeater section foroptimum transmission conditions by controlling the tunable light source106 based on the temperature evaluation value. FIG. 28 shows an exampleof a repeaterless transmission system, and FIG. 29 an example of anoptical amplification repeater transmission system. FIG. 30 is concernedwith an example where the prechirping amount α is corrected based on thetemperature evaluation value.

FIGS. 31 to 33 are concerned with an example where the dispersioncompensation amount is corrected. An optical amplification repeatertransmission system is shown here, but it will be recognized that theexample is also applicable to a repeaterless transmission system. FIG.31 shows a configuration where the variable dispersion compensator 101is installed in the transmitter, FIG. 32 concerns a configuration wherethe variable dispersion compensator 101 is installed in the receiver,and FIG. 33 illustrates a configuration where the variable dispersioncompensator 101 is installed in both the transmitter and receiver, andalso in every repeater.

FIG. 34 is concerned with an example in which the waveform degradationdue to the SPM-GVD effect is suppressed by correcting transmitter powerand repeater optical output power based on the temperature evaluationvalue. Instead of controlling the amplifier, the light source may becontrolled. FIG. 35 is concerned with an example where signal lightwavelength, prechirping amount, dispersion compensation amount, andoptical power are corrected.

The processing for these corrections may be performed manually or may beautomatically performed by the CPU. Furthermore, a separate CPU may beprovided for each of the regenerative repeater sections between theoptical transmitter and receiver for independent control, oralternatively, control may be performed centrally by a single CPU whileadjusting the relationships among the plurality of regenerative repeatersections.

It is believed that the SPM effect is caused by abrupt changes in fiberrefractive index due to abrupt changes in light intensity. Therefore, bytransmitting optical pulses with the rise time and fall time forcefullystretched and thereby smoothing the variation of optical signalstrength, the waveform degradation due to the SPM effect can be reduced.In this case, rather than causing only the light intensity to varysmoothly and stretching the rising/falling transition times, it would bepreferable to stretch the transition times by deliberately causingwavelength dispersion, since it would then be possible to compensate forit by dispersion compensation or other means in a later stage.Deliberate wavelength dispersion can be caused either by intentionallyshifting the signal light wavelength from the zero dispersion wavelengthλ₀ and thereby causing dispersion by GVD, or by inserting a dispersioncompensator in the transmitter.

FIGS. 36 and 37 are concerned with an example in which the wavelengthλ_(s) of signal light is set to a value spaced apart from the zerodispersion wavelength λ₀ of DSF and a dispersion compensator 112 with afixed dispersion amount is installed at the receiving end. FIG. 36 showsan example of repeaterless transmission, and FIG. 37 an example ofrepeaterless transmission, and FIG. 37 an example of multiple repeatertransmission. The dispersion amount D of the dispersion compensator 112is set to a value that can compensate for the GVD caused by λ_(s) ≠λ₀.

FIGS. 38 and 39 are concerned with an example where the dispersioncompensator 112 is installed at the transmitted end. FIG. 38 shows anexample of repeaterless transmission, and FIG. 39 an example of multiplerepeater transmission. In this case also, the dispersion amount D of thedispersion compensator 112 is set to a value that can compensate for theGVD caused by λ_(s) ≠λ₀.

In the example of FIG. 36 or 37, if the system is set expressly forλ_(s) <λ₀ and D>0, as shown in FIG. 40, since λ_(s) is in the negativedispersion region, four-wave mixing between signal light and spontaneousemission can be prevented. Of course, a combination of λ_(s) >λ₀ andD<0, as shown in FIG. 41, can also serve the purpose. Furthermore,dispersion compensators with dispersion value D of opposite signs may beinstalled at the transmitter and receiver, respectively, as shown inpart (a) and part (b) of FIG. 42. It is also possible to install adispersion compensator at the transmitter, at the receiver, and at everyrepeater or some of the repeaters as shown in FIG. 43.

Further optimization can be achieved by measuring the transmissioncharacteristic with the wavelength λ_(s) of signal light first set at avalue spaced apart from the zero dispersion wavelength λ₀ and with thedispersion compensator arranged at an appropriate location to suppressthe SPM effect, as described above, and then by correcting λ_(s) to anoptimum value based on the result of the measurement. FIGS. 44 and 46show several examples of such a system configuration. For the method ofmeasuring the transmission characteristic and the mode of control, anyof the various methods and modes already described can be applied. Asshown in FIGS. 47 to 49, the prechirping amount may be controlled withthe signal light wavelength λ_(s) fixed. Alternatively, both the signallight wavelength λ_(s) and the prechirping amount may be controlled, asshown in FIGS. 50 to 52. Control of the prechirping amount can beaccomplished using a Mach-Zehnder optical modulator, as previouslydescribed.

In the examples thus far described, a single signal light wavelength wasused throughout one regenerative repeater section between the opticaltransmitter and receiver, regardless of the provision of an opticalamplification repeater. In the examples hereinafter described, awavelength converter is installed in every optical amplificationrepeater and the signal light wavelength λ_(s) is optimized for eachamplification repeater section.

FIG. 53 shows an example of an optical transmission system in which awavelength converter 118 is provided in every optical amplificationrepeater and the signal light wavelength is optimized for eachamplification repeater section. In the example of FIG. 53, thewavelength converter 118 is provided for every amplification repeatersection, but the invention is not limited to this configuration.Furthermore, a variable wavelength light source 106 may be provided inthe transmitter, as shown in FIG. 54, to further optimize the signallight wavelength for the section between the transmitter and the firstoptical amplification repeater.

The wavelength converter 118 can be implemented using a wavelengthconverting laser constructed, for example, from a bistable optical lasersuch as shown in FIG. 55. The left-hand half of the figure is the regionof optical bistability; the electrode above the active layer 120 isdivided into two sections, and the portion between them is used as asaturable absorption region. When input light is injected while thedevice is held in a state about to oscillate by adjusting currents ingain regions 122 and 124, the saturable absorption region 126 becomestransparent and starts lasing, producing output lights at differentwavelengths. The right-hand half of the figure is the control region foroscillation wavelength, which consists of a phase shift region 128 and aDBR region 130 having a diffraction grating 129. When current isinjected into the DBR region 130, the refractive index of a light guidelayer 132 decreases because of the plasma effect caused by carriers,thus making it possible to shift the Bragg wavelength toward the shorterwavelength side. Further, by varying the current being injected to thephase shift region 128, an equivalent light path length of this regioncan be varied, which makes it possible to match the phase of light tothe oscillation condition. Therefore, by suitably varying the currentsapplied to these two regions, the wavelength of output light can becontrolled over a wide range.

In a second implementation of the wavelength converter 112, thephenomenon of four-wave mixing is deliberately made use of. When lightat two wavelengths, λ₀ and λ_(in), near the zero dispersion wavelength,is input to the DSF, light of λ_(out) =λ₀ +(λ₀ +λ_(in)) is produced as aresult of four-wave mixing. When a tunable light source is used to emitλ₀ which is made variable, and only light at λ_(out) is extracted fromthe output light by using a filter, λ_(in) can be converted to λ_(out)while controlling its wavelength.

By setting signal light wavelength for each optical amplificationrepeater section, it becomes possible to further increase thetransmission speed, since wavelength dispersing can be further reduced,and the transmission line cost can be reduced since the allowable rangefor the variation of the zero dispersion wavelength λ₀ can be expanded.Furthermore, by wavelength-converting high-speed optical signals aslight at each optical amplification repeater, rather than regeneratingand repeating them and setting the signal light wavelength once againfor further transmission, two conversions between optical and electricalsignals and associated high-speed electronic circuitry can be omitted,thus achieving reductions in the size and cost of the system.

When the zero dispersion wavelength of the transmission line, includingits variation along the longitudinal direction, is already known, thesignal light wavelength is set at an optimum value for each opticalamplification repeater section through simulation, etc. On the otherhand, when the zero dispersion wavelength of the transmission line isnot known, the transmission characteristic is measured at the receivingend when the system is started up, while sweeping the wavelength by thetunable light source and wavelength converter, and the wavelength is setfor optimum transmission characteristic. It would also be possible tosweep the wavelength while feeding back the control signal from thetransmission characteristic measuring section 105, as shown in FIG. 56.In this case, each wavelength converter is first set to zero wavelengthshift amount, and the tunable light source is operated to performsweeping to determine the wavelength that provides the best transmissioncharacteristic. If the transmission characteristic does not meet thespecification at this time, then the wavelength converters could beoperated to perform sweeping, for example, in sequence first startingwith the wavelength converter closest to the transmitter, then the nextclosest one, and so on, setting each to the wavelength that provides thebest transmission characteristic. For the method of measuring thetransmission characteristic and the mode of control during systemstartup and during system operation, any of the various methods andmodes already described can be applied.

In the example shown in FIG. 57, a dispersion compensator 112 fordeliberately causing GVD is provided in the transmitter to reduce theSPM effect as already described. The dispersion compensator may also beinstalled at each repeater.

Next, peripheral techniques for implementing optical multiplexing willbe described.

The Mach-Zehnder optical modulator, used to produce an optical signal bymodulating a light beam from a light source with an electrical signal,has a sinusoidal characteristic as previously described with referenceto FIG. 20, but since the characteristic drifts with changingtemperature and aging, the drift must be compensated for so that thevariation range (operating point) of the applied voltage is always heldwithin specified limits. Japanese Patent Unexamined Publication No.3-251815 discloses a technique whereby the drift of a Mach-Zehndermodulator is compensated for by amplitude-modulating the applied voltage(high-frequency electrical signal) with low-frequency signal offrequency f₀ and by controlling the bias of the applied voltage so thatthe f₀ component contained in the output light becomes zero. That is,when the driving voltage range V₀ -V₁ is within the proper limits, theupper and lower envelopes of the output light signal vary with frequency2f₀, their phases being opposite from each other, as shown in FIG. 58,so that no f₀ component is contained; on the other hand, when theoperating point has shifted outside the proper range, the upper andlower envelopes of the output light signal vary with frequency f₀ and inphase with each other, as shown in FIGS. 59 and 60, so that the f₀component is contained. To prevent this, part of the output light signalis separated using a coupler and converted to an electrical signal, andthe bias of the optical modulator is controlled using an outputphase-detected at f₀, to stabilize the operating point.

When such a drift compensation technique is applied to an opticalmultiplexing system, since each optical channel requires the provisionof an optical modulator, an equal number of drift compensation circuitsneed to be provided. Accordingly, if the above drift compensationtechnique is directly applied to an optical multiplexing system, therearises the problem that couplers for splitting optical signals andoptical detectors for converting separated optical signals intoelectrical signals become necessary in large numbers.

FIG. 61 shows an example of an optical multiplexing system equipped witha drift compensation circuit according to the present invention. In thisexample, laser beams of the same wavelength λ₀ are input to a pluralityof Mach-Zehnder optical modulators 201₁, 201₂, . . . , arranged inparallel, and drive circuits, 203₁, 203₂, . . . , for the opticalmodulators 201₁, 201₂, . . . , amplitude-modulate drive signals(modulation signals) with low-frequency signals of different frequenciesf₁, f₂, . . . , generated by low-frequency oscillators, 204₁, 204₂, . ...

Output lights from the optical modulators, 201₁, 201₂, . . . , areoptically combined and transmitted onto an optical transmission line,while part of the combined light is separated as monitor light by usingan optical coupler 205; the separated light is then converted to anelectrical signal and further divided into a plurality of signals whichare passed through band filters 208₁, 208₂, . . . , and applied tocorresponding phase-detection/bias-supply circuits 202₁, 202₂, . . . Theband filter 208_(k) (k=1, 2, . . . , the same applies hereinafter)transmits frequency f_(k) of the low-frequency-superimposed componentsof the corresponding optical modulator 201_(k).

The phase-detection/bias-supply circuit 202_(k) phase-detects thelow-frequency component of the output light extracted by the band filter208_(k) with the output of the oscillator 204_(k), and generates asignal to control the operating point of the optical modulator 201_(k).This control is performed simultaneously in all the optical modulators201₁, 201₂, . . .

In the above configuration, the phase-detection/bias-supply circuit 202₁for the optical modulator 201₁ is controlled using the low-frequencycomponent f₁ extracted by the band filter 208₁, and likewise, thephase-detection/bias-supply circuit 202₂ for the optical modulator 201₂is controlled using the low-frequency component f₂ extracted by the bandfilter 208₂. Accordingly, bias control for each of the opticalmodulators, 201₁, 201₂, . . . , arranged in parallel, can be performedindependently of each other.

The above configuration is effective when performing opticaltime-division multiplexing (OTDM) of a plurality of optical signals. Byperforming output light separation and optical/electrical conversion atone location, simultaneous control of a plurality of optical modulatorscan be accomplished. In this example, the band filters, 208₁, 208₂, . .. , are used to extract the corresponding frequency components afteroptical/electrical conversion and separation, but if stable operationcan be ensured, these filters may be omitted.

In the example of FIG. 61, control of the operating point drift bylow-frequency amplitude modulation is performed simultaneously in allthe optical modulators, 201₁, 201₂, . . . , operating in parallel.Alternatively, the system may be constructed in such a manner that eachof the drive circuits for performing low-frequency amplitude modulationis switched into operation in turn at certain intervals of time so thatat any given time only one drive circuit is performing low-frequencyamplitude modulation; in that case, the operating point drift only ofthe optical modulator currently performing the low-frequency modulationis detected and controlled, while the operating points of the otheroptical modulators are held fixed. In this manner, signals of the samefrequency can be used as the low-frequency signals.

FIG. 62 shows an example of such an optical multiplexing system. In thisexample, control is switched, at fixed intervals of time T₀, betweenoptical modulators arranged in parallel. More specifically, a pluralityof Mach-Zehnder optical modulators, 201₁, 201₂, . . . , are arranged inparallel; first, optical signals of the same wavelength λ₀ are modulatedby the optical modulators, 201₁, 201₂, . . . , and then, the modulatedsignals are combined together. Only one low-frequency generator 204 isprovided that generates a single low frequency f₀ which is supplied viaa selector switch 209 to the drive circuits, 203₁, 203₂, . . . , each ofwhich is switched into operation in turn at fixed intervals of time T₀.Each drive circuit thus switched into operation in turn at fixed timeintervals performs low-frequency amplitude modulation with the singlefrequency f₀.

On the output side of the optical modulators, 201₁, 201₂, . . . , thecombined output light is split by the optical coupler 205 and convertedby the optical detector 206 into an electrical signal which is suppliedto the phase-detection/bias-supply circuit 202. Thephase-detection/bias-supply circuit 202 phase-detects the low-frequencycomponent in the electrical signal converted from the separated outputlight with the low-frequency f₀ signal from the low-frequency oscillator204, and generates a bias voltage for output.

The output of the phase-detection/bias-supply 202 is supplied via aselector switch 210 to the optical modulators 201₁, 201₂, . . . Theselector switch 210, which operates in interlocking fashion with theselector switch 209, controls the operating point drift by supplying thebias voltage only to the optical modulator whose associated drivecircuit is performing low-frequency amplitude modulation. During thattime, the operating points of the other optical modulators are heldfixed (for example, using a latch or the like: the same applieshereinafter).

This example, as with the example of FIG. 61, is effective when opticaltime-division multiplexing a plurality of optical signals, and has anadditional advantage that control can be accomplished using onephase-detection/bias supply-circuit. To prevent occurrence of drifts inthe optical modulators not under control, the time T₀ is set as short aspossible but sufficiently longer than the time constant of control.

FIG. 63 shows another example of the optical multiplexing system of theinvention. In this example, the optical modulators 201₁, 201₂, . . . ,are arranged in series. That is, the system is constructed with aplurality of Mach-Zehnder optical modulators 201₁, 201₂, . . . ,arranged in series so that the light of wavelength λ₀ from the lightsource undergoes modulation two or more times. This system should not becalled an optical multiplexing system because optical signals are notmultiplexed. In this specification, however, the system is called anoptical multiplexing system, for convenience.

The drive circuits, 203₁, 203₂, . . . , for the optical modulators,201₁, 201₂, . . . , are constructed to perform low-frequency amplitudemodulation with respectively different frequencies f₁, f₂, . . . Theoutput light from the final-stage optical modulator is split by theoptical coupler 205 and converted by the optical detector 206 into anelectrical signal which is supplied to the phase-detection/bias-supplycircuits, 202₁, 202₂, . . . , via the respective band filters 208₁,208₂, . . . The band filter 208_(k) transmits the frequency f_(k) of thelow-frequency-superimposed components of the corresponding opticalmodulator 201_(k).

The phase-detection/bias-supply circuit 202_(k) phase-detects thelow-frequency component in the signal separated from the output lightwith the low-frequency f_(k) signal from the oscillator 204_(k), andthus detects an operating point drift and controls the operating pointof the corresponding optical modulator 201_(k). Control of the operatingpoint is performed simultaneously in all the optical modulators, 201₁,201₂, . . . If stable operation can be ensured, the band filters, 208₁,208₂, . . . , may be omitted.

In the example of FIG. 63, control of the operating point drift usinglow-frequency superimposition is performed simultaneously in all theoptical modulators, 201₁, 201₂, . . . Alternatively, the system may beconstructed in such a manner that each of the drive circuits forperforming low-frequency amplitude modulation is switched into operationin turn at certain intervals of time so that at any given time only onedrive circuit is performing low-frequency amplitude modulation; in thatcase, the operating point drift only of the optical modulator currentlyperforming the low-frequency modulation is detected and controlled,while the operating points of the other optical modulators are heldfixed.

FIG. 64 shows an example of such an optical multiplexing system. In thisexample, control is switched, at fixed intervals of time T₀, betweenoptical modulators, 201₁, 201₂, . . . , arranged in series. Morespecifically, a plurality of Mach-Zehnder optical modulators, 201₁,201₂, . . . , are arranged in series to construct a system in whichlight from the light source undergoes modulation two or more times. Onlyone low-frequency generator 204 is provided that generates a single lowfrequency f₀ which is supplied via the selector switch 209 to the drivecircuits, 203₁, 203₂, . . . , each of which is switched into operationin turn at fixed intervals of time T₀. Each drive circuit thus switchedinto operation in turn at fixed time intervals performs low-frequencyamplitude modulation with the single frequency f₀.

The output light from the final-stage optical modulator is split by theoptical coupler 205 and converted by the optical detector 206 into anelectrical signal which is supplied to the phase-detection/bias-supplycircuit 202. The phase-detection/bias-supply circuit 202 phase-detectsthe low-frequency component in the electrical signal converted from theseparated output light with the single low-frequency f₀ signal from thelow-frequency oscillator 204, and generates a bias voltage for output.

The output of the phase-detection/bias-supply 202 is supplied via theselector switch 210 to the optical modulators 201₁, 201₂, . . . Theselector switch 210, which operates in interlocking fashion with theselector switch 209, controls the operating point drift by supplying thebias voltage only to the optical modulator whose associated drivecircuit is performing low-frequency amplitude modulation. During thattime, the operating points of the other optical modulators are heldfixed.

This example, as with the example of FIG. 63, is effective for opticaltime-division multiplexing, and has an additional advantage that controlcan be accomplished using one phase-detection/bias-supply circuit.

FIG. 65 shows another example of the optical multiplexing system of theinvention. In this example, the optical modulators, 201₁, 201₂, . . . ,arranged in parallel, modulate lights of respectively differentwavelengths, λ₁, λ₂, . . . , which are then wavelength-multiplex. Thedrive circuits, 203₁, 203₂, . . . , for the optical modulators, 201₁,201₂, . . . , perform low-frequency amplitude modulation withrespectively different frequencies f₁, f₂, . . . That is, a plurality ofMach-Zehnder optical modulators, 201₁, 201₂, . . . , are arranged inparallel to construct a system in which optical signals of differentwavelengths, λ₁, λ₂, . . . , are wavelength-multiplexed. The drivecircuits, 203₁, 203₂, . . . , for the optical modulators, 201₁, 201₂, .. . , perform low-frequency amplitude modulation with respectivelydifferent frequencies f₁, f₂, . . . , and the optical outputs from theoptical modulators, 201₁, 201₂, . . . , are combined together to producewavelength-multiplexed output light.

The wavelength-multiplexed output light is split by the optical coupler205 and converted by the optical detector 206 into an electrical signalwhich is supplied to the phase-detection/bias-supply circuits, 202₁,202₂, . . . , via the respective band filters 208₁, 208₂, . . . The bandfilter 208_(k) transmits the frequency f_(k) of thelow-frequency-superimposed components of the corresponding opticalmodulator 201_(k). The phase-detection/bias-supply circuit 202_(k)phase-detects the low-frequency component in the signal split from theoutput light with the low-frequency f_(k) signal from the oscillator204_(k), and thus detects an operating point drift and controls theoperating point of the optical modulator 201_(k). This control isperformed simultaneously in all the optical modulators, 201₁, 201₂, . ..

This example is effective for wavelength multiplexing, and allowssimultaneous control of a plurality of optical modulators, in the sameprinciple as that of the example shown in FIG. 61. If stable operationcan be ensured, the band filters, 208₁, 208₂, . . . , may be omitted.

In the example of FIG. 65, control of the operating point drift usinglow-frequency amplitude modulation is performed simultaneously in allthe optical modulators, 201₁, 201₂. . . Alternatively, the system may beconstructed in such a manner that each of the drive circuits forperforming low-frequency amplitude modulation is switched into operationin turn at certain intervals of time so that at any given time only onedrive circuit is performing low-frequency amplitude modulation; in thatcase, the operating point drift only of the optical modulator currentlyperforming the low-frequency modulation is detected and controlled,while the operating points of the other optical modulators are heldfixed.

FIG. 66 shows an example of such an optical multiplexing system. In thisexample, control is switched, at fixed intervals of time T₀, betweenoptical modulators, 201₁, 201₂, . . . , arranged in parallel and towhich optical signals of different wavelengths, λ₁, λ₂, . . . , areinput. More specifically, a plurality of Mach-Zehnder opticalmodulators, 201₁, 201₂, . . . , are arranged in parallel; first opticalsignals of different wavelengths λ₁, λ₂, . . . are modulated by therespective optical modulators, 201₁, 201₂, . . . , and then, themodulated signals are combined together. Only one low-frequencygenerator 204 is provided that generates a single low frequency f₀ whichis supplied via the selector switch 209 to the drive circuits, 203₁,203₂, . . . , each of which is switched into operation in turn at fixedintervals of time T₀. Each drive circuit thus switched into operation inturn at fixed time intervals performs low-frequency amplitude modulationwith the single frequency f₀.

On the output side of the optical modulators, 201₁, 201₂, . . . , thecombined output light is split by the optical coupler 205 and convertedby the optical detector 206 into an electrical signal which is suppliedto the phase-detection/bias-supply circuit 202. Thephase-detection/bias-supply circuit 202 phase-detects the low-frequencycomponent in the electrical signal converted from the separated outputlight with the single low-frequency f₀ signal from the low-frequencyoscillator 204, and generates a bias voltage for output.

The output of the phase-detection/bias-supply 202 is supplied via theselector switch 210 to the optical modulators 201₁, 201₂, . . . Theselector switch 210, which operates in interlocking fashion with theselector switch 209, controls the operating point drift by supplying thebias voltage only to the optical modulator whose associated drivecircuit is performing low-frequency amplitude modulation. During thattime, the operating points of the other optical modulators are heldfixed.

This example, as with the example of FIG. 65, is effective forwavelength multiplexing, and has an additional advantage that controlcan be accomplished using one phase-detection/bias-supply circuit.

FIG. 67 shows another example of the optical multiplexing systemaccording to the present invention. In this example, optical outputsfrom the optical modulators, 201₁, 201₂, . . . , arranged in paralleland to which optical signals of different wavelengths λ₁, λ₂, . . . ,are input, are combined together to produce combined output light whichis then separated by a wavelength separating device 212 into the signalsof wavelengths, λ₁, λ₂, . . . , as originally supplied from the opticalmodulators 201₁, 201₂, . . . , before converting them to electricalsignals. That is, a plurality of Mach-Zehnder optical modulators, 201₁,201₂, . . . , are arranged in parallel to construct a system in whichoptical signals of respectively different wavelengths, λ₁, λ₂, . . . ,are wavelength-multiplexed. Only one low-frequency oscillator 204 isprovided that generates a single low frequency f₀. The drive circuits,203₁, 203₂, . . . , perform low-frequency amplitude modulation with thesingle frequency f₀.

Optical outputs from the optical modulators, 201₁, 201₂, . . . , arecombined together to produce wavelength-multiplexed output light. Thiswavelength-multiplexed output light is split by the optical coupler 205and is passed through the wavelength separating device 212 forseparation into the optical signals of wavelengths λ₁, λ₂, . . . Theseseparated optical signals are fed to the optical detectors, 206₁, 206₂,. . . , for conversion into electrical signals which are supplied to thephase-detection/bias-supply circuits 202₁, 202₂, . . . , respectively.The phase-detection/bias-supply circuits 202_(k) phase-detects thelow-frequency component in the wavelength-separated signal with thelow-frequency f₀ signal, and thus detects an operating point drift andcontrols the operating point of the corresponding optical modulator201_(k). This control is performed simultaneously in all the opticalmodulators 201₁, 201₂, . . . This example, as with the example of FIG.65, is effective for wavelength multiplexing, and is particularlyapplicable where light wavelength provides better separability.

FIG. 68 shows another example of the optical multiplexing systemaccording to the present invention. In this example, control isswitched, at fixed intervals of time T₀, between optical modulators,201₁, 201₂, . . . , arranged in parallel and to which lights ofdifferent wavelengths, λ₁, λ₂, . . . , are input, and using a tunablefilter 213 whose transmission wavelength varies with time, thewavelength component only of the optical modulator currently undercontrol is extracted.

More specifically, a plurality of Mach-Zehnder optical modulators, 201₁,201₂, . . . , are arranged in parallel to construct a system in whichoptical signals of different wavelengths, λ₁, λ₂, . . . , arewavelength-multiplexed. Only one low-frequency oscillator 204 isprovided that generates a single low frequency f₀. The drive circuits,203₁, 203₂, . . . , perform low-frequency amplitude modulation with thesignal frequency f₀.

Optical outputs from the optical modulators, 201₁, 201₂, . . . , arecombined together to produce wavelength-multiplexed output light. Thiswavelength-multiplexed output light is split by the optical coupler 205,passed through the tunable optical filter 213, and converted by theoptical detector 206 into an electrical signal. The tunable opticalfilter 213 is a filter whose transmission wavelength varies with time,and at any given time, extracts only one wavelength component foroutput. An output signal from the optical detector 206 is supplied via aselector switch 214 to phase-detection/bias-supply circuits, 202₁, 202₂,. . . , each of which is switched into operation in turn at fixedintervals of time. The selector switch 214 operates in interlockingfashion with the tunable optical filter 213 so that when the tunableoptical filter 213 is switched so as to transmit wavelength λ_(k), theselector switch 214 is set so as to supply its output signal to thephase-detection/bias supply circuit 202_(k).

This phase-detection/bias-supply circuit 202_(k) phase-detects thelow-frequency component in the optical/electrical converted signal withthe low-frequency f₀ signal, and detects and controls the operatingpoint drift of the optical modulator 201_(k) corresponding to theextracted wavelength λ_(k). during that time, the operating points ofthe other optical modulators are held fixed.

This example, as with the example of FIG. 65, is effective forwavelength multiplexing, and is particularly applicable where wavelengthselection using a tunable optical filter is easier than using othermeans. In this example, the drive circuits for performing low-frequencymodulation need not be switched in sequence at fixed time intervals, butsuch switching may be performed to ensure clear separation of thelow-frequency component appearing as a result of the operating pointdrift of each optical modulator.

FIG. 69 shows another example of the optical multiplexing systemaccording to the present invention. The optical multiplexing systemshown here is constructed by adding sign-inverting circuits 215_(k) and216_(k), describe din Japanese Unexamined Patent Publication No.4-140712, to the system shown in FIG. 61. As described with reference toFIGS. 20 and 21, in the Mach-Zehnder optical modulator the direction ofprechirping can be changed from red shift to blue shift by changing theoperating point from Vb1 to Vb2. When the wavelength of the signal lightis in the normal dispersion region of the DSF, red shift is provided,and when it is in the abnormal dispersion region, blue shift isprovided, to improve the waveform. The sign-inverting circuit 215_(k)changes the operating point from Vb1 to Vb2 by inverting, in accordancewith an operating point switching signal, the polarity of thelow-frequency signal being applied from the oscillator 204_(k) to thedrive circuit 203_(k). When the operating point is changed to Vb2, thelogic of the optical signal are reversed, so that the logic of themodulation signal is inverted by the sign-inverting circuit 216_(k) insynchronism with the switching operation of the sign-inverting circuit215_(k). Instead of inverting the logic of the signal supplied from theoscillator 204_(k) to the drive circuit 203_(k), the logic of the signalsupplied from the oscillator 204_(k) to the phase-detection/bias-supplycircuit 202_(k), or the result of phase detection in thephase-detection/bias-supply circuit 202_(k), may be inverted.Furthermore, the switching of the operating point may be performedsimultaneously for all the optical modulators 201_(k).

In the example of FIG. 69, sign-inverting circuits are added to theoptical multiplexing system of FIG. 61, but the invention is not limitedto this particular example. Rather it will be appreciated that thesign-inverting circuits can be added likewise to any of the opticalmultiplexing systems thus far described (FIGS. 62 to 68), in which casethe switching operation of the sign-inverting circuits may be performedindividually or together in interlocking fashion.

Changing the operating point by the sign-inverting circuit(operating-point shift circuit) can be implemented in several ways; forexample, an external selector switch may be provided, or automaticswitching may be performed in the system by checking the transmissioncharacteristic, for example, at the receiving end. When transmitting ata wavelength near the zero dispersion wavelength of the fiber, the signof wavelength dispersion during transmission can become positive ornegative, depending on the variation of the zero dispersion wavelengthof the fiber, the variation of the light source wavelength, etc. In suchcases, it may be useful if the operating point in each optical modulatoris changed independently. In the case of wavelength multiplexing, whenthe relationship of magnitude between the zero dispersion wavelength ofthe fiber and each signal light wavelength is already known, it may beuseful if the operating points in the optical modulators having the samerelationship of magnitude are changed together at a time. n the case ofoptical time-division multiplexing also, the chirping of the outputlight wavelength can be reversed by changing the operating points of theserially arranged optical modulators at the same time.

Next, a clock extraction technique in optical time-division multiplexing(OTDM) will be described.

FIG. 70 hows an optical time-division multiplexing transmission systemin which the clock signal extraction according to the invention isapplied. This embodiment is concerned with a system configuration thatachieves a transmission rate of 40 Gb/s by two-wave multiplexing. FIG.71 is a timing chart for various signals used in the receiver of thissystem, showing the waveforms of the signals designated by referencesigns, a to i, in FIG. 70.

First, using a one-input, two-output optical switch 241 that operates ona single 20-GHz sine wave b, 20-GHz optical signals, c an d, whosephases are opposite from each other, are created from an optical signal,a, output from a light source LD (laser diode) 240. The optical clocksignals, c and d, are then applied to external modulators 244 and 245,respectively, where they are externally modulated by 20-Gb/s NRZsignals, e and f, producing 20-Gb/s RZ optical signals g and h. Then,these signals are bit-multiplexed (optical MUX) by a wave combiner 246which outputs a 40-Gb/s optical multiplexed signal i. With this opticaltime-division multiplexing (OTDM) method, 40-Gb/s optical transmissioncan be achieved without requiring an ultra-wideband electronic devicecapable of 40 Gb/s.

In an alternative configuration, a short-pulse light source or an LDwith a semiconductor optical modulator may be used instead of the lightsource LD 240 and optical switch 241 shown in FIG. 70, or the beamsplitting optical switch 241 at the transmitting end may be replaced bya simple optical power splitting device or an external modulator whichis driven by a sine wave.

At the receiving end, on the other hand, the 40-Gb/s optical multiplexedsignal, i, must be demultiplexed into two 20-Gb/s RZ optical signal(optional DEMUX). Recent years have seen many proposals and experimentson optical DEMUX techniques using ultra-fast PLLs utilizing nonlineareffects such as four-wave mixing and cross-phase modulation (XPM), butany of the proposed techniques requires large-scale circuitry andfurthermore, there are stability problems yet to be overcome.

The simplest method that can be considered is therefore by performingbit-demultiplexing alternately on a bit-by-bit basis using a one-input,two-output optical switch such as the one used in the transmitter shownin FIG. 70. In FIG. 70, the optical multiplexed signal received from thetransmission line 240 is input to a bit-demultiplexing optical switch252 via an optical preamplifier 249, while part of it is separated by anoptical coupler 250 and input to a clock extraction circuit 251. In theclock extraction circuit 251, as shown in FIG. 82 for example, the inputsignal is first converted to electrical form by an optical detector 260,and then a clock signal is directly extracted using a narrowbandelectrical filter (dielectric resonance filter, SAW filter, etc.) 262.The extracted clock signal is applied to the optical switch 252 as asignal for providing bit-demultiplexing timing. In synchronism with thisclock signal, the optical switch 252 demultiplexes the received 40-Gb/soptical multiplexed signal, i, into two 20-Gb/s RZ optical signals(optical DEMUX) and supplies them to respective optical receivers 253and 254.

In this receiver configuration, however, a 20-GHz clock signalsynchronized to the data main signal is required not only for theidentification of the sign but for the optical switching operation ofthe optical switch 252; therefore, the received optical multiplexedsignal needs to contain a 20-GHz component.

In the present invention, a 20-GHz component of a magnitude sufficientfor the extraction of the clock signal is carried on the transmittedoptical multiplexed signal, i, in the following way. That is, as shownin FIG. 72, at the transmitter the two RZ signals, g and h, aregenerated with different amplitudes, and the clock signal is extractedfrom the resulting 40-GHz optical multiplexed signal i. As shown, theoptical multiplexed signal, i, thus produced carries a 20-GHz clocksignal component of sufficient magnitude as shown by a dotted line inthe figure.

The following describes various methods for providing the opticalsignals with different amplitudes which are multiplexed to produce anoptical multiplexed signal containing a clock signal component. Forconvenience of explanation, the following description deals with twocases: case A where separate light source LDs are used whose opticaloutputs are first modulated externally and then combined together, andcase B where output light from one light source LD is split and theresulting signals are combined after external modulation, as in theexample of FIG. 70.

FIG. 74 shows one example of case A. Separate light source LDs areprovided that supply respective optical signals to external modulators244 and 245; if these light source LDs, 240a and 240b, are set todifferent output powers, the optical signals, g and h, to be multiplexedcan be provided with different amplitudes.

FIG. 75 shows another example of case A. As shown, by inserting anoptical attenuator 256 in one or other of the light paths leading fromthe light source LDs to the wave combiner 246, the optical signals, gand h, to be multiplexed can be provided with different amplitudes. Inthe illustrated example, the optical attenuator 256 is inserted betweenthe external modulator 244 and the wave combiner 246, but it may beinserted between the light source LD 240a and the external modulator244. Of course, the optical attenuator 256 may be inserted in the lightpath where the external modulator 245 is located. It would be possibleto use an optical amplifier instead of the optical attenuator 256.Furthermore, these methods are applicable not only to externalmodulation but also to LD direct modulation or direct modulation using amodulator-incorporated LD.

FIG. 76 shows an example in which the method of FIG. 75 is applied tocase B. This example of case B is identical to the foregoing example,except that only one light source LD is used, and therefore, detailedexplanation is not given here.

In either case A or B, in an optical time-division multiplexingtransmission system using external modulation, light intensity amplitudedifferences between a plurality of optical signals can be realized byusing external modulators, 244 and 245, having different insertionlosses.

When a Mach-Zehnder optical modulator is used as the external modulator,the amplitude of the output light can be changed by changing theamplitude of the voltage for driving the optical modulator or bychanging its bias point. FIGS. 77 and 78 are diagrams for explaining howthis can be achieved. FIG. 77 shows how the light output strengthchanges when the amplitude value of the drive voltage (applied voltage)is changed from Ve to Vf. FIG. 78 shows how the light output intensitychanges when the bias voltage of the drive voltage is changed from VB-eto VB-f. In this way, as long as a Mach-Zehnder light modulator is usedfor each external modulator, the output light intensities of theexternal modulators 244 and 245 can be changed by changing the drivevoltage amplitude or the bias voltage.

Furthermore, in case B, it is also possible to provide the opticaloutputs of the external modulators with different amplitudes by settingthe split ratio of the output light of the light source LD 240, in theoptical switch 241 (or a passive optical power splitting device that canbe used in its place), to a value other than 1:1.

FIG. 79 shows another example of case B. In this example, the externalmodulators 244 and 245 are set so that their optical outputs arelinearly polarized with their principal axes of polarizationperpendicular to each other. When optical-multiplexing the two RZsignals having different polarization conditions, for example, withtheir principal axes of linear polarization at right angles to eachother, as described above, if a polarization-dependent optical device257 is inserted in a light path after the optical multiplexing in thetransmitter (wave combiner 246), the multiplexed two optical signalswill emerge from the optical device 257 with their amplitudes of lightintensity different from each other, thus producing an opticalmultiplexed signal with its light amplitude varying between alternatebits.

Instead of inserting the polarization-dependent optical device 257 asdescribed above, it would be possible to replace the wave combiner by awave combiner whose combining ratio is dependent on polarization eitherstructurally or by the incident polarization axes of the optical signal.

Furthermore, in a transmission system in which the polarizationrelationship between alternate bits at the transmitter is maintained toa certain degree through to the receiver, a polarization-dependentoptical device 258 may be inserted before the optical demultiplexing inthe optical switch 252 in the receiver, as shown in FIG. 80.

This embodiment has described two-wave multiplexing having a differencein light intensity amplitude between alternate bits, but an N-wavemultiplexing configuration is also possible. For example, in the case ofa four-wave optical multiplexing transmission system also, a clocksignal can be extracted from an optical multiplexed signal. FIG. 73shows an example of such four-wave multiplexed are designated as g1, g2,g3, and g4, amplitude differences are set so that the relationg1>g2=g4>g3 holds. When these signals are combined to produce an opticalmultiplexed signal i, the optical multiplexed signal, i, will contain aclock signal component as shown by a dotted line in the figure. Further,in this example, it is possible to produce a multiplexed signalcontaining a plurality of clock signal components, depending on how theamplitude differences are set.

Another characteristic of the invention lie sin the following point.That is, in conventional optical transmission systems at transmissionrates up to 10 Gb/s, clock extraction is performed by splitting the mainsignal in the electrical stage after detecting (converting to electricalform) the signal light. By contrast, in the present invention, using oneof the above-described methods a clock signal is extracted from theoptical multiplexed signal separated from the main signal in the opticalstage, and optical demultiplexing is performed using the thus extractedclock signal.

Another problem that arises in the multiplexing transmission system isthat it is generally required that the correspondence between thechannels at the transmitter before multiplexing and the channels at thereceiver after demultiplexing be predetermined in fixed fashion. Forexample, in FIG. 70, it is required that the signal supplied to thedrive circuit 242 be always received by the optical receiver 253 and thesignal supplied to the drive circuit 243 be always received by theoptical receiver 254. However, in conventional OTDM transmissionsystems, since channel identification is not done at the receiver, thecorrespondence may change each time the system is started, makingtransmission line management impossible.

FIG. 83 shows the configuration of an optical demultiplexser suitablefor use at the receiving end of the OTDM communication system accordingto the present invention. The optical demultiplexer comprises an opticalcoupler 300 for splitting the received optical signal into two signals,a clock signal regenerating circuit 302 for regenerating a clock signalfrom one of the splitted signals, an optical switch 304 for separatingthe received optical signal as optical signal corresponding to eachoptical signal channel by using the regenerated clock signal, and twooptical receivers, 306 and 308, each for recovering data from theoptical signal on each separated channel. This configuration isfundamentally the same as that of the receiving section of the systemshown in FIG. 70.

Data on each optical signal channel is structured, for example, in theformat shown in FIG. 84. Data on each optical signal channel areoptically multiplexed by bit-interleaving, and transmitted from thetransmitter. In FIG. 84, reference numeral 310 indicates framesynchronization data used to establish frame synchronization at theoptical receiver 306 or 308, and 312 designates identification data foridentifying the channel. Line identification data extraction circuits,314 and 316, extract the identification data 312; in accordance with theidentification data extracted by the line identification data extractioncircuits 314 and 316, a control circuit 318 controls a signal switchingcircuit 320 to control the connection in the signal switching circuit320 so that the data intended for an output line 1 is output onto theoutput line 1 and the data for an output line 2 is output onto theoutput line 2. Input signals to the line identification data extractioncircuits 314 and 316 may be derived from the outputs of the signalswitching circuit 320. The control circuit 318 is easily implementedusing a microprocessor.

In the optical demultiplexer shown in FIG. 85, instead of switching theoutput connection in each optical receiver the phase of the clock signalapplied to the optical switch 304 is changed by controlling a phaseshifter 322, thereby essentially achieving the same effect as connectionswitching. In the case of two-wave multiplexing, switching between theconnections can be accomplished essentially by shifting the phase of theclock signal by 180°.

FIG. 86 shows an example in which the optical demultiplexer of FIG. 85is expanded from the two-channel to the four-channel configuration. Theclock signal reconstructed by the clock signal generating circuit 302 issupplied to the optical switch 304 via the phase change 322. The clocksignal is also supplied to a divide-by-2 frequency divider 303 where itis divided by 2, and the divided signal is then supplied to opticalswitches 305 and 305' via phase changes 323 and 323', respectively.Assuming that four-channel optical signals, CH1 to CH4, are multiplexedin the order of CH1, CH2, CH3, and CH4, as shown n part (a) of FIG. 115,since the optical switch 304 is switched for each time slot by a clocksignal shown in part (b) of FIG. 115, CH1 and CH3 are output from oneoutput as shown in part (c) of FIG. 115 and CH2 and CH4 are output fromthe other output as shown in part (d) of FIG. 115 in alternatingfashion. Since the optical switches 305 and 305' are each switched forevery two time slots by a clock signal shown in part (e) and part (h) ofFIG. 115, CH1 and CH3 are separated as shown in part (f) and (g) of FIG.115 and CH2 and CH4 are separated as shown in part (i) and part (j) ofFIG. 115, respectively. In accordance with the identification dataextracted by the line identification data extraction circuits 314 to317, the control circuit 318 controls the phase shifters 322, 323, and323', for example, in such a manner that CH1 is output from output line1, CH2 from output line 2, CH3 from output line 3, and CH4 from outputline 4.

While the phase of the clock signal is shifted by 180° by the phaseshifter 322 in FIG. 85, in the optical demultiplexer shown in FIG. 87the optical signal before demultiplexing is delayed or advanced throughan optical delay circuit 324 by the amount of time equivalent to a 180°phase shift of the clock signal. Instead of changing the phase of theclock signal, the same effect can be obtained by delaying or advancingthe optical signal by the amount of time equivalent to it. The opticaldelay circuit 324 can be implemented using, for example, a corner cube326, as shown in FIG. 88, which is moved by mechanical means to changethe light path length.

FIG. 89 shows another example of the optical demultiplexer of thepresent invention. In the example of FIG. 89, low-frequency signals, f₁-f₄, of different frequencies for different channels, are superimposed,as shown in FIG. 90, for channel identification, instead of suing theidentification data 312 in FIG. 84. FIG. 89 shows a case of two-wavemultiplexing.

The optical signals separated by the optical switch 304 are split in therespective optical couplers 328 and 330, and converted by optical/electrical conversion circuits 332 and 334 into electrical signalswhich are supplied to low-frequency detectors 336 and 338 for detectionof the low-frequency signals superimposed on the respective signals. Thecontrol circuit 318 identifies the channels from the frequencies of thelow-frequency signals and detected by the low-frequency detectors 336and 338, and switches the connections in the signal switching circuit320 so that the signals intended for the respective channels are outputon the respective output lines 1 and 2. As described earlier, instead ofswitching the connections the phase of the clock signal may be changedas sown in FIG. 91, or the optical signal may be delayed or advanced asshown in FIG. 92. Further, in the case of two-wave multiplexing, sinceit is sufficient to identify one or the other of the two channels, theoptical coupler 330, the optical/electrical conversion circuit 334, andthe low-frequency detector 338 need not necessarily be provided, andonly one channel may be used in the event of failure of that onechannel. Further, the optical receiver 306 or 308 may be provided with alight current monitor circuit 342 for monitoring the current flowing toan optical/electrical converting element 340, as shown in FIG. 93, inwhich case the low-frequency signal can be derived form its output. Inthis case, the optical couplers, 328 and 330, and the optical/electricalconversion circuits, 332 and 334, can be eliminated.

FIG. 94 shows the configuration of an optical transmitter fortransmitting optical signals by superimposing thereon low-frequencysignals f₁ of different frequencies for different channels. Light from alight source 400 is "punched" in an external optical modulator 402 insynchronism with a clock signal, and optical pulse trains are produced,and split by an optical coupler 404 into a required number of outputs(two in the case shown in the figure). The first split light ismodulated with a frequency f₁ in an external optical modulator 406, andfurther modulated with a first main signal in an external modulator 408.Likewise, the second split light is modulated with a frequency f₂, andfurther modulated with a second main signal in an external opticalmodulator 412; the thus modulated light is passed through an opticaldelay element 414 to produce a time shift with respect to the firstoptical signal, and added together in an adder 416. Consequently, duringthe period (time slot) that the light is modulated with the first mainsignal, the frequency f₁ is superimposed in amplitude, and during thetime slot in which the light is modulated with the second main signal,the frequency f₂ is superimposed in amplitude. It is desirable that thesignal "punching" in the external optical modulator 402 be performed sothat the resulting multiplexed signal will have the waveform shown inFIG. 107, the pulse for each channel just occupying one time slot. Theadvantage obtained by this will be explained later.

In the optical multiplexing system shown in FIG. 94, the input opticalsignal is split without introducing a phase difference between the splitsignals, and after applying modulation, a phase shift is introduced andthe two signals are combined together; by contrast, in the opticalmultiplexing system shown in FIG. 70, the input optical signal is splitby the optical switch 241 into two signals with opposite phases, andthese signals are directly combined together. In the latter system also,if an external optical modulator for modulating with the frequency f₁ orf₂ is provided in series to the external optical modulators 244 and 245,low-frequency signals of different frequencies for different channelscan be superimposed. Each of the external optical modulators, 402, 406,408, 410, and 412, can be implemented using a Mach-Zehnder opticalmodulator or an electric-field absorption optical modulator (EA opticalmodulator).

FIG. 95 shows another example of the optical transmitter of the presentinvention. The number of external optical modulators can be reduced byalready superimposing the frequencies f₁ and f₂ in amplitude on therespective main signals in drive circuits 418 and 420. The drivecircuits 418 and 420 each can be implemented using a dual-gate FET suchas shown in FIG. 96. A waveform in drive circuits are shown in FIG. 120.

The optical demultiplexer for the optical receiver explained withreference to FIG. 83 to 92 can be modified to use it as a some kind ofoptical exchange that exchanges optical signals according toidentification information included in an optical multiplexed signal.For example, by replacing the optical receiver 306 and 308 of FIG. 85and FIG. 87 with optical couplers 600 and 602, respectively, as shown inFIG. 116 and FIG. 117, and by connecting another outputs of the opticalcouplers 600 and 602 to optical outputs 1 and 2, the modified opticaldemultiplexer can be used as an optical exchange. The circuit of FIG. 91or FIG. 92 can be modified to an optical exchange by directly connectingthe output of the optical couplers 328 and 330 to optical output line 1and 2, as shown in FIG. 118 or FIG. 119.

When low-frequency signals are superimposed on the received signal, asshown in FIG. 90, the low-frequency signals can be used not only forchannel identification but also for control for stabilizing the phase ofthe clock signal used for optical demultiplexing. FIG. 97 shows theconfiguration of an optical receiver having a phase controller forstabilizing the clock signal by using the superimposed low-frequencysignals.

The optical receiver shown comprises an optical coupler 430 forsplitting the received Q-bit optical signal into two signals, a timingregenerator 432 for regenerating a Q/2-Hz clock signal from one of thesplit signals, an optical switch 434 for separating the optical signalas light into two Q/2-bit/s optical signals by using the regeneratedclock signal, and optical receivers 436 and 438 for recovering the datasignals from the separated optical signals. This configuration isfundamentally the same as that shown in FIG. 83.

The phase of the clock signal supplied from the timing regenerator 432to the optical switch 434 is changed by a phase shifter 439 which isunder control of a phase controller 440. The phase controller 440comprises an optical coupler 442 for splitting one of the separatedQ/2-bit/s optical signals, an optical detector 444 for converting one ofthe separated optical signals into an electrical signal, a band-passfilter 446 for transmitting only a signal of designated frequency f₁ inthe output from the optical detector 444, an oscillator 448 with anoscillation frequency of g₁, a synchronous detection circuit 450 forperforming phase synchronous detection on the output of the band-passfilter 446 with a signal of frequency g₁, a comparator 452 for comparingthe detection output of the synchronous detection circuit 450 with apredetermined reference value and for generating a control voltage basedon the result of the comparison, and an adder 454 for adding the outputof the comparator 452 to the output of the oscillator 448 and foroutputting a control signal for the phase shifter 439. Low-frequencysignals of different frequencies need not be superimposed on the opticalsignals of all channels, but need only be superimposed on a designatedchannel. In the latter case, the ban-pass filter 446 can be eliminated.However, in the latter case also, if the band-pass filter 446 isinserted, the S/N ratio of the signal input to the synchronous detectioncircuit 450 can be improved.

It is assumed here that the frequency f₁ is superimposed only on CH1, asshown in FIG. 98(a). When the phase of the clock signal perfectlymatches the phase of the optical signal in the optical switch 434, asshown in FIG. 98(b), the strength of the signal of frequency f1 outputfrom the optical detector 444 becomes the greatest, as shown in FIG.98(c). On the other hand, when the clock signal is out of phase, asshown in FIG. 98(d), since the f₁ signal is not derived in tis entiretyin the optical switch 434, the strength of the f₁ signal decreases. Morespecifically, as the phase difference θ between the optical signal andthe clock signal, as defined in FIG. 99, changes from 0° to ±180°, thestrength of the f₁ component decreases linearly, as shown in FIG. 100.Since the phase shifter 409 is controlled by the output of theoscillator 448, the clock signal is slightly phase-modulated with thefrequency g₁. Suppose here that the center of the phase variation is atpoint (b) in FIG. 100. Then, since the strength of the f₁ componentvaries with the frequency g₁ and the straight line is upward to theright, the variation of the strength of the f₁ component is in phase or180° out of phase with the frequency g₁ output from the oscillator 448.Therefore, when the phase synchronous detection is done in thesynchronous detection circuit 450 with the frequency g₁, the outputtakes a positive or negative value of the same magnitude. When thecenter of the phase variation is at point (c) in FIG. 100, since thestraight line is downward to the right, the phase detection output takesthe same value as when at point (b) but the sign is inverted. When thecenter is at point (a), since half of the variation is folded back,there is no variation component of the frequency g₁ and the phasedetection output becomes zero. Therefore, by comparing the output of thesynchronous detection circuit 450 with the reference value, for example,the zero level, in the comparator 452, and by controlling the phase ofthe clock signal with the sum of the result of the comparison and theoutput of the oscillator 448, the operating point can be controlled topoint (a) in FIG. 100. The control signal being input to the comparator452 is for inverting the polarity of the output of the comparator 452;when the polarity of the output of the comparator 452 is inverted, thecenter of the control shifts form point (a) (maximum) to minimum, thusshifting the phase of the clock signal by 180°. Switching between thechannels can be easily achieved in this manner.

The circuit shown in FIG. 101 is a modified example of the opticalreceiver of FIG. 97. In the circuit of FIG. 101, not only the opticalsignal to the optical receiver 438 is split, but the optical signal tothe optical receiver 436 is split by an optical coupler 460, and inputto a phase controller 562 having the same configuration as the phasecontroller 440. However, while the center frequency of the band-passfilter in the phase controller 440 is f₁, in the phase controller 462the center frequency of the band-pass filter is f₂ to achieve locking onthe other channel. The outputs of the band-pass filters in the phasecontrollers 440 and 462 are fed to a comparator 464 for comparison witha reference value. The enables the detection of any channel not in use.When one of the channels is not in use, a selector 466 is controlled toselect the output signal of one or other of the phase controllers 440and 462 and output it to control the phase of the clock signal using thephase shift amount control signal obtained for the channel in use. Thephase shift amount control signal selected by the selector 466 issupplied to the phase shifter 439. If there is more than one channelidentified as being in use, the selector 466 is controlled according topredetermined priority.

FIG. 102 also shows a modified example of FIG. 97. Instead of splittingthe optical signal and converting it to an electrical signal fordetection of low-frequency signals in the phase controller 440, theoptical receiver 438 includes a light current monitor circuit 472 for anoptical-electrical converting element 470, and low-frequency signals arederived from the output of the light current monitor circuit 472, asdescribed with reference to FIG. 93.

FIG. 103 shows an example of a configuration expanded to four-wavemultiplexing. Two Q/2-bit/s optical signals separated by the opticalswitch 434 are further separated by optical switches 474 and 479 intofour Q/4-bit/s optical signals. The Q/2-Hz clock signal output from thephase shifter 439 is divided by a divide-by-2 frequency divider 478, andthe resulting Q/4-Hz clock signal is supplied to the optical switches474 and 476 via phase shifters 480 and 482, respectively. The phasecontroller 440 controls the phase of the Q/2-Hz clock signal based onfrequency f₁. Since the frequency f₁ is superimposed, for example, onCH1 and CH3 time slots, as shown in FIG. 104, the Q/2-Hz clock signal isphase synchronized to the CH1 or CH3 time slot under the control of thephase controller 440. This ensures stable separation of CH1+CH3 andCH2+CH4 by the optical switch 434. It should b noted that f₁ need notnecessarily be superimposed on CH3.

In the example of FIG. 103, it is assumed that CH1+CH3 is input to theoptical switch 476 and CH2+CH4 to the optical switch 474. A phasecontroller 484 controls the phase of the Q/4-Hz clock signal input tothe optical switch 476, based on frequency f₂. In the example shown inFIG. 104, the frequency f₂ is superimposed on the CH1 time slot, so thatthe optical switch 476 can separate CH1 and CH3 stably. A phasecontroller 486 controls the phase of the Q/4-Hz clock signal input tothe optical switch 474, based on frequency f₃. In the example shown inFIG. 104, the frequency f₃ is superimposed on the CH2 time slot, so thatthe optical switch 474 can separate CH2 and CH4 stably.

FIG. 103 shows an example for four-wave multiplexing. A system foreight-wave or 16-wave multiplexing can be constructed by cascading anoptical switch, divider, and phase controller in a similar manner.

In a modified example of the circuit of FIG. 103, the phase shifters 480and 482 and their associated phase controllers 484 and 486 can beomitted, as shown in FIG. 105, provided that the phases of the opticalpath and electrical path are adjusted at the time of manufacture so thatthe phase of the Q/4-Hz clock signal applied to the optical switches 474and 476 becomes optimum if the Q/2-Hz clock signal applied to theoptical switch 434 is optimum. In this configuration, since only thelow-frequency signal f₁ needs to be superimposed, the band-pass filter446 also can be omitted.

Furthermore, the configuration of FIG. 103 may be modified so that thefrequencies, f₁, f₂, and f₃, necessary for the phase controllers, 440,484, and 486, can be detected from the output of the light currentmonitor circuit of the optical/electrical converting element provided inthe optical receiver, as in the example shown in FIG. 102. In this case,provisions must be made so that f₁ and f₂ are detected from the opticalreceiver 453 that is to receive CH1, and f₃ from the optical receiver455 that is to receive CH3.

FIG. 106 shows an example of the detailed configuration of the timingregenerator 432 and phase shifter 439 shown in FIGS. 96, 101, 102, 103,and 105. The timing regenerator 432 comprises an optical detector 490for performing optical/electrical conversion, a nonlinear extractioncircuit 492 for deriving a Q-Hz component from the output of the opticaldetector 490, a timing filter 494 for extracting only the Q-Hz componentfrom the output of the nonlinear extraction circuit 492, a limiteramplifier 496 for keeping the output of the timing filter 494 at aconstant amplitude, and a divide-by-2 frequency divider 498. It isadvantageous to insert the phase shifter 439 between the timing filter494 and the limiter amplifier 496.

Further, the nonlinear extraction circuit 492 comprises a differentialcircuit 500 and a full-wave rectification circuit 502. When signalswitch each pulse positioned within one time slot are opticalmultiplexed, the differential circuit 500 and full-wave rectificationcircuit 502 for performing nonlinear extraction can be omitted, thussimplifying the timing generator 432.

In the thus simplified configuration, the optical detector 490 forperforming optical/electrical conversion may be constructed to have aresonant frequency at Q Hz as shown in FIG. 108. This makes thefabrication of the optical detector easier, since the detector need notto have a flat frequency characteristic over a wide range. Furthermore,since the detector has a filter characteristic, the out-of-bandattenuation characteristic of the timing filter 494 can be eased.

FIG. 109 shows a modified example of the circuit of FIG. 106. A couplercircuit 504 is used to separate a portion of a signal, and the level ofthe received signal is detected by a level detector 506. The detectedlevel is compared in a comparator 508 with a reference value, and if thelevel is below the reference value, an input off alarm signal is output.Since the circuit from the input to the comparator 508 is made up onlyof passive components which are less prone to failure, the input-offcondition of the light can be detected reliably. With thisconfiguration, if a signal-off condition has occurred in the opticalreceiver, the cause of that condition can be identified, that is,whether the optical input signal has been cut off or has gone out ofsynchronization.

The optical switch for separating an input optical switch into two 180°out-of-phase optical signals by using a clock signal can be implementedby a directional active optical coupler using a Ti-diffused LiNbO₃crystal waveguide described in Japanese Patent Unexamined PublicationNo. 55-7315. As shown in FIG. 110, it can also be constructed using twogate-type optical switches 510 and 512, an optical coupler circuit 514for distributing optical signals to them, and a phase shifter 516 forshifting the phase of the clock signal input to the gate-type opticalswitch 510 by 180°. This configuration can reduce polarizationdependence than the directional active optical coupler. The gate-typeoptical switch is a device that exhibits varying transmittance with anapplied voltage and therefore can also be used as an optical modulator.An electric-field absorption optical modulator (EA modulator) is anexample.

In any of the optical receivers described with reference to FIGS. 97 to105, an optimum clock signal was obtained by controlling the phase ofthe clock signal recovered from the received optical signal. Instead, itwould be possible to generate a clock signal using a voltage-controlledoscillator (VCO) and control it so that the frequency and phase becomeoptimum, as shown in FIG. 111. In FIG. 111, the phase controller 440 isidentical in configuration and operation to the one shown in FIG. 97.The clock signal supplied to the optical switch 434 is generated by theVCO 520, while the control signal output from the phase controller 440is given to the VCO 520.

The following describes how the clock signal generated by the VCO 520 iscontrolled to the optimum value.

First, definitions are given as follows.

φ(t): Phase of the clock signal input to the optical switch 434 (phaseof the output signal from the VCO 520)

α(t): Phase of the optical signal input to the optical switch (forsimplicity, it is assumed that an alternating signal 1010 is input) withrespect to channel 1

θ(t): φ(t)-α(t) (see FIG. 99)

ω₀ : Free-running angular frequency of VCO

V(t): Control voltage for VCO×Angular frequency modulation sensitivityof VCO

Vo: Output value of synchronous detection circuit 450

K: Constant

The strength of the f₁ component that passes through the band-passfilter 446 (center frequency f₁) varies with the phase difference θ asshown in FIG. 100.

Here, perturbation is applied to θ by adding a low-frequency signal, I₀cos2πg₁ t, output from the oscillator 448, to the control voltageapplied to VCO 520.

    dφ/dt=ω.sub.0 +V(t)=ω.sub.0 +I.sub.0 cos2πg.sub.1 t+KVo(1)

Now, Vo is assumed to be changing slowly or in a steady state, andtherefore constant.

Integrating equation (1) gives equation (2), from which it can be seenthat φ i phase-modulated with the frequency g₁.

    φ=(ω.sub.0 +KVo)t+I.sub.1 sin2πg.sub.1 t+C

    (C: Constant of integration, I.sub.1 : Constant)           (2)

Therefore, assuming that the derivative of α(t) with respect to time isconstant, θ(t) is phase-modulated with the frequency g₁ as in the caseof FIG. 97.

When θ is phase-modulated, the f₁ component transmitted through theband-pass filter 446 exhibits the response shown in FIG. 100, as alreadydescribed. That is, at point (a) where the waveform is folded back, thecomponent of sin2πg₁ t does not exist in the variation of the f₁component. At point (b), it is in phase with the input signal, and atpoint (c), it is 180° out of phase.

Therefore, the output Vo(t) of the synchronous detection performed usingthe output of oscillator 448, I₀ sin2πg₁ t, will be as shown in FIG.112.

(It is assumed that +1 is output when in phase and -1 when 180° out ofphase.)

Next, the convergence of the operation is observed. When theperturbation component is removed for simplicity, this can be written as

    dφ/dt=ω.sub.0 +V(t)=ω.sub.0 +KVo(t)        (3)

Transforming this equation, we have

    dθ/dt=dφ/dt-dα/dt=ω.sub.0 -dα/dt+Vo(t)=Δω.sub.0 +KVo(t)

Since dα/dt is a constant value, Δω₀ is a constant value. If K ispositive and is large, and if ←ω₀ /K≈0, then

when θ>0, Vo(t)<0 and dθ/dt<0, and hence θ converges to 0, and

when θ<0, Vo(t)>0 and dθ/dt>0, and hence θ converges to 0.

From the above, it is shown that θ converges to 0, which indicates thatthe optical signal can be switched between the light paths by theoptical switch with optimum timing.

FIG. 113 shows a circuit configuration for implementing the detection ofa light input-off condition in the receiver configuration shown in FIG.111. The received optical signal is split by an optical coupler 522 oneof whose output is supplied to the optical switch 434 (FIG. 111). Theother output is supplied to an optical detector 524 for conversion intoan electrical signal. This electrical signal is input to band-passfilters 526 and 528 for detection of the low-frequency components f₁ andf₂, respectively, which are then normalized by being divided by a DCvalue in dividers 530 and 532, and are compared with a reference valuein comparators 534 and 536, and the results of the comparisons are ANDedto produce an input off alarm signal. The circuit shown in FIG. 111assumes that f₁ is superimposed on one channel and f₂ on the otherchannel. Only one of f₁ and f₂ may be monitored, but when both aremonitored, only one of the lines can be used. The dividers 530 and 532are provided to remove the effects of input power variation, but thesecan be omitted when the input power is stable.

FIG. 114 shows an expanded version of the circuit of FIG. 111 forfour-wave multiplexing. Clock signals applied to optical switches 540and 542 are created by dividing the output of the VCO 520 by two in adivide-by-two frequency divider 544. The low-frequency signal f₁ issuperimposed on CH1 and CH2.

We claim:
 1. An optical transmission system comprising:opticaltime-division multiplexing means for time-division multiplexing aplurality of optical signal channels; means for appending identificationinformation for identifying each optical signal channel, to an opticalmultiplexed signal generated by the optical time-division multiplexingmeans; an identification information extraction circuit for extractingthe identification information contained in the optical signal channel;and a control circuit for changing output destinations so that eachoptical signal channel is output on a designated destination inaccordance with the identification information extracted by theidentification information extraction circuit.
 2. An opticaltransmission system according to claim 1, further comprising:a clockregenerating circuit for regenerating a clock signal for each opticalsignal channel from the optical multiplexed signal; and an opticalswitch for separating each optical signal channel from the opticalmultiplexed signal in accordance with the clock signal regenerated bythe clock regenerating circuit.
 3. An optical transmission systemaccording to claim 2, whereinthe identification information appendingmeans appends identification data to a data signal transmitted on eachof the optical signals, and the identification information extractioncircuit extracts the identification data contained in the data signalreconstructed from each optical signal channel.
 4. An opticaltransmission system according to claim 3, wherein the control circuitchanges output destinations by changing connections between the outputof each optical signal channel and the output destinations.
 5. Anoptical transmission system according to claim 3, wherein the controlcircuit changes output destinations by changing the phase of the clocksignal supplied to the optical switch.
 6. An optical transmission systemaccording to claim 3, wherein the control circuit changes outputdestinations by delaying or advancing the optical multiplexed signalinput to the optical switch.
 7. An optical transmission system accordingto claim 2, whereinthe identification information appending meanssuperimposes a low-frequency signal on the optical multiplexed signal ina time slot designated for a specific optical signal channel, and theidentification information extraction circuit extracts theidentification information by detecting the low-frequency signalsuperimposed on the optical multiplexed signal.
 8. An opticaltransmission system according to claim 7, wherein the identificationinformation extraction circuit detects the low-frequency signal from anelectrical signal converted from each optical signal channel separatedby the optical switch.
 9. An optical transmission system according toclaim 7, wherein the identification information extraction circuitdetects the low-frequency signal from a current flowing to anoptical/electrical converting device for converting each optical signalchannel separated by the optical switch into an electrical signal. 10.An optical transmission system according to claim 7, further comprisinga signal switching circuit provided between outputs of the opticalswitch and output destinations, wherein the control circuit changesoutput destinations by changing in the signal switching circuitconnection relationships between the outputs of the optical switch andthe output destinations.
 11. An optical transmission system according toclaim 7, wherein the control circuit changes output destinations bychanging the phase of the clock signal supplied to the optical switch.12. An optical transmission system according to claim 7, wherein thecontrol circuit changes output destinations by delaying or advancing theoptical multiplexed signal input to the optical switch.
 13. An opticaltransmitter comprising:optical time-division multiplexing means fortime-division multiplexing a plurality of optical signal channels; andmeans for appending identification information for identifying eachoptical signal channel, to an optical multiplexed signal generated bythe optical time-division multiplexing means.
 14. An optical transmitteraccording to claim 13, wherein the identification information appendingmeans superimposes a low-frequency signal on the optical multiplexedsignal in a time slot designated for a specific optical signal channel.15. An optical transmitter according to claim 14, wherein theidentification information appending means includes a second opticalmodulator which is connected in series to an optical modulator forgenerating the specific optical signal channel, and to which thelow-frequency signal is applied as a modulating signal.
 16. An opticaltransmitter according to claim 14, wherein the identificationinformation appending means includes a drive circuit for superimposingthe low-frequency signal on a modulating signal for the specific opticalsignal channel, and for supplying the resulting signal as a modulatingsignal to an optical modulator for generating the optical signalchannel.